15 Oracle Dynamic SQL: Method 4

What Makes Method 4 Special?

Unlike Methods 1, 2, and 3, dynamic SQL Method 4 lets your program

• Accept or build dynamic SQL statements that contain an unknown number of select-list items or placeholders, and

• Take explicit control over datatype conversion between Oracle and C types

To add this flexibility to your program, you must give the Oracle runtime library additional information.

What Information Does Oracle Need?

The Pro*C/C++ Precompiler generates calls to Oracle for all executable dynamic SQL statements. If a dynamic SQL statement contains no select-list items or placeholders, Oracle needs no additional information to execute the statement. The following DELETE statement falls into this category:

DELETE FROM emp WHERE deptno = 30 

However, most dynamic SQL statements contain select-list items or placeholders for bind variables, as does the following

UPDATE statement:

UPDATE emp SET comm = :c WHERE empno = :e
 

To execute a dynamic SQL statement that contains placeholders for bind variables or select-list items, Oracle needs information about the program variables that hold the input (bind) values, and that will hold the FETCHed values when a query is executed. The information needed by Oracle is:

• The number of bind variables and select-list items

• The length of each bind variable and select-list item

• The datatype of each bind variable and select-list item

• The address of each bind variable, and of the output variable that will receive each select-list item

Where Is the Information Stored?

All the information Oracle needs about select-list items or placeholders for bind variables, except their values, is stored in a program data structure called the SQL Descriptor Area (SQLDA). The SQLDA struct is defined in the sqlda.h header file.

Descriptions of select-list items are stored in a select descriptor, and descriptions of placeholders for bind variables are stored in a bind descriptor.

The values of select-list items are stored in output variables; the values of bind variables are stored in input variables. You store the addresses of these variables in the select or bind SQLDA so that Oracle knows where to write output values and read input values.

How do values get stored in these data variables? Output values are FETCHed using a cursor, and input values are typically filled in by the program, usually from information entered interactively by the user.

How is the SQLDA Referenced?

The bind and select descriptors are usually referenced by pointer. A dynamic SQL program should declare a pointer to at least one bind descriptor, and a pointer to at least one select descriptor, in the following way:

#include <sqlda.h>
... 
SQLDA *bind_dp; 
SQLDA *select_dp; 

You can then use the SQLSQLDAAlloc() function to allocate the descriptor, as follows:

bind_dp = SQLSQLDAAlloc(runtime_context, size, name_length, ind_name_length); 

SQLSQLDAAlloc() was known as sqlaldt() before Oracle8.

The constant SQL_SINGLE_RCTX is defined as (dvoid*)0. Use it for runtime_context when your application is single-threaded.

How is the Information Obtained?

You use the DESCRIBE statement to help obtain the information Oracle needs.

The DESCRIBE SELECT LIST statement examines each select-list item to determine its name and name length. It then stores this information in the select SQLDA for your use. For example, you might use select-list names as column headings in a printout. The total number of select-list items is also stored in the SQLDA by DESCRIBE.

The DESCRIBE BIND VARIABLES statement examines each placeholder to determine its name and length, then stores this information in an input buffer and bind SQLDA for your use. For example, you might use placeholder names to prompt the user for the values of bind variables.

Purpose of the SQLDA

Method 4 is required for dynamic SQL statements that contain an unknown number of select-list items or placeholders for bind variables. To process this kind of dynamic SQL statement, your program must explicitly declare SQLDAs, also called descriptors. Each descriptor is a struct which you must copy or code into your program.

A select descriptor holds descriptions of select-list items, and the addresses of output buffers where the names and values of select-list items are stored.

A bind descriptor holds descriptions of bind variables and indicator variables, and the addresses of input buffers where the names and values of bind variables and indicator variables are stored.

Multiple SQLDAs

If your program has more than one active dynamic SQL statement, each statement must have its own SQLDA(s). You can declare any number of SQLDAs with different names. For example, you might declare three select SQLDAs named sel_desc1, sel_desc2, and sel_desc3, so that you can FETCH from three concurrently OPEN cursors. However, non-concurrent cursors can reuse SQLDAs.

Declaring a SQLDA

To declare a SQLDA, include the sqlda.h header file. The contents of the SQLDA are:

struct SQLDA 
{ 
    long    N;          /* Descriptor size in number of entries */ 
    char  **V;        Ptr to Arr of addresses of main variables */ 
    long   *L;              /* Ptr to Arr of lengths of buffers */ 
    short  *T;                /* Ptr to Arr of types of buffers */ 
    short **I;      * Ptr to Arr of addresses of indicator vars */ 
    long    F;         /* Number of variables found by DESCRIBE */ 
    char  **S;          /* Ptr to Arr of variable name pointers */ 
    short  *M;       /* Ptr to Arr of max lengths of var. names */ 
    short  *C;    * Ptr to Arr of current lengths of var. names */ 
    char  **X;         /* Ptr to Arr of ind. var. name pointers */ 
    short  *Y;  /* Ptr to Arr of max lengths of ind. var. names */ 
    short  *Z;  /* Ptr to Arr of cur lengths of ind. var. names */ 
}; 

Allocating a SQLDA

After declaring a SQLDA, you allocate storage space for it with the SQLSQLDAAlloc() library function (known as sqlaldt() before Oracle8), using the syntax:

descriptor_name = SQLSQLDAAlloc (runtime_context, max_vars, max_name, max_ind_name); 

where:

Besides the descriptor, SQLSQLDAAlloc() allocates data buffers to which descriptor variables point.

Figure 15-1 shows whether variables are set by SQLSQLDAAlloc() calls, DESCRIBE commands, FETCH commands, or program assignments.

Figure 15-1 How Variables Are Set

Description of the illustration lnpcc010.gif

The N Variable

N specifies the maximum number of select-list items or placeholders that can be DESCRIBEd. Thus, N determines the number of elements in the descriptor arrays.

Before issuing the optional DESCRIBE command, you must set N to the dimension of the descriptor arrays using the SQLSQLDAAlloc() library function. After the DESCRIBE, you must reset N to the actual number of variables DESCRIBEd, which is stored in the F variable.

The V Variable

V is a pointer to an array of addresses of data buffers that store select-list or bind-variable values.

When you allocate the descriptor, SQLSQLDAAlloc() zeros the elements V[0] through V[N - 1] in the array of addresses.

For select descriptors, you must allocate data buffers and set this array before issuing the FETCH command. The statement

EXEC SQL FETCH ... USING DESCRIPTOR ... 

directs Oracle to store FETCHed select-list values in the data buffers to which V[0] through V[N - 1] point. Oracle stores the ith select-list value in the data buffer to which V[i] points.

For bind descriptors, you must set this array before issuing the OPEN command. The statement

EXEC SQL OPEN ... USING DESCRIPTOR ...
 

directs Oracle to execute the dynamic SQL statement using the bind-variable values to which V[0] through V[N - 1] point. Oracle finds the ith bind-variable value in the data buffer to which V[i] points.

The L Variable

L is a pointer to an array of lengths of select-list or bind-variable values stored in data buffers.

For select descriptors, DESCRIBE SELECT LIST sets the array of lengths to the maximum expected for each select-list item. However, you might want to reset some lengths before issuing a FETCH command. FETCH returns at most n characters, where n is the value of L[i] before the FETCH.

The format of the length differs among Oracle datatypes. For CHAR or VARCHAR2 select-list items, DESCRIBE SELECT LIST sets L[i] to the maximum length of the select-list item. For NUMBER select-list items, scale and precision are returned respectively in the low and next-higher bytes of the variable. You can use the library function SQLNumberPrecV6() to extract precision and scale values from L[i]. See also "Extracting Precision and Scale".

You must reset L[i] to the required length of the data buffer before the FETCH. For example, when coercing a NUMBER to a C char string, set L[i] to the precision of the number plus two for the sign and decimal point. When coercing a NUMBER to a C float, set L[i] to the length of floats on your system. For more information about the lengths of coerced datatypes, see also "Converting Data".

For bind descriptors, you must set the array of lengths before issuing the OPEN command. For example, you can use strlen() to get the lengths of bind-variable character strings entered by the user, then set the appropriate array elements.

Because Oracle accesses a data buffer indirectly, using the address stored in V[i], it does not know the length of the value in that buffer. If you want to change the length Oracle uses for the ith select-list or bind-variable value, reset L[i] to the length you need. Each input or output buffer can have a different length.

The T Variable

T is a pointer to an array of datatype codes of select-list or bind-variable values. These codes determine how Oracle data is converted when stored in the data buffers addressed by elements of the V array.

For select descriptors, DESCRIBE SELECT LIST sets the array of datatype codes to the internal datatype (CHAR, NUMBER, or DATE, for example) of the items in the select list.

Before FETCHing, you might want to reset some datatypes because the internal format of Oracle datatypes can be difficult to handle. For display purposes, it is usually a good idea to coerce the datatype of select-list values to VARCHAR2 or STRING. For calculations, you might want to coerce numbers from Oracle to C format.

The high bit of T[i] is set to indicate the NULL/not NULL status of the ith select-list item. You must always clear this bit before issuing an OPEN or FETCH command. You use the library function SQLColumnNullCheck() to retrieve the datatype code and clear the NULL/not NULL bit.

You should change the Oracle NUMBER internal datatype to an external datatype compatible with that of the C data buffer to which V[i] points.

For bind descriptors, DESCRIBE BIND VARIABLES sets the array of datatype codes to zeros. You must set the datatype code stored in each element before issuing the OPEN command. The code represents the external (C) datatype of the data buffer to which V[i] points. Often, bind-variable values are stored in character strings, so the datatype array elements are set to 1 (the VARCHAR2 datatype code). You can also use datatype code 5 (STRING).

To change the datatype of the ith select-list or bind-variable value, reset T[i] to the datatype you want.

The I Variable

I is a pointer to an array of addresses of data buffers that store indicator-variable values.

You must set the elements I[0] through I[N - 1] in the array of addresses.

For select descriptors, you must set the array of addresses before issuing the FETCH command. When Oracle executes the statement

EXEC SQL FETCH ... USING DESCRIPTOR ... 

if the ith returned select-list value is NULL, the indicator-variable value to which I[i] points is set to -1. Otherwise, it is set to zero (the value is not NULL) or a positive integer (the value was truncated).

For bind descriptors, you must set the array of addresses and associated indicator variables before issuing the OPEN command. When Oracle executes the statement

EXEC SQL OPEN ... USING DESCRIPTOR ... 

the data buffer to which I[i] points determines whether the ith bind variable has a NULL value. If the value of an indicator variable is -1, the value of its associated bind variable is NULL.

The F Variable

F is the actual number of select-list items or placeholders found by DESCRIBE.

F is set by DESCRIBE. If F is less than zero, DESCRIBE has found too many select-list items or placeholders for the allocated size of the descriptor. For example, if you set N to 10 but DESCRIBE finds 11 select-list items or placeholders, F is set to -11. This feature lets you dynamically reallocate a larger storage area for select-list items or placeholders if necessary.

The S Variable

S is a pointer to an array of addresses of data buffers that store select-list or placeholder names as they appear in dynamic SQL statements.

You use SQLSQLDAAlloc() to allocate the data buffers and store their addresses in the S array.

DESCRIBE directs Oracle to store the name of the ith select-list item or placeholder in the data buffer to which S[i] points.

The M Variable

M is a pointer to an array of maximum lengths of data buffers that store select-list or placeholder names. The buffers are addressed by elements of the S array.

When you allocate the descriptor, SQLSQLDAAlloc() sets the elements M[0] through M[N - 1] in the array of maximum lengths. When stored in the data buffer to which S[i] points, the ith name is truncated to the length in M[i] if necessary.

The C Variable

C is a pointer to an array of current lengths of select-list or placeholder names.

DESCRIBE sets the elements C[0] through C[N - 1] in the array of current lengths. After a DESCRIBE, the array contains the number of characters in each select-list or placeholder name.

The X Variable

X is a pointer to an array of addresses of data buffers that store indicator-variable names. You can associate indicator-variable values with select-list items and bind variables. However, you can associate indicator-variable names only with bind variables. So, X applies only to bind descriptors.

Use SQLSQLDAAlloc() to allocate the data buffers and store their addresses in the X array.

DESCRIBE BIND VARIABLES directs Oracle to store the name of the ith indicator variable in the data buffer to which X[i] points.

The Y Variable

Y is a pointer to an array of maximum lengths of data buffers that store indicator-variable names. Like X, Y applies only to bind descriptors.

You use SQLSQLDAAlloc() to set the elements Y[0] through Y[N - 1] in the array of maximum lengths. When stored in the data buffer to which X[i] points, the ith name is truncated to the length in Y[i] if necessary.

The Z Variable

Z is a pointer to an array of current lengths of indicator-variable names. Like X and Y, Z applies only to bind descriptors.

DESCRIBE BIND VARIABLES sets the elements Z[0] through Z[N - 1] in the array of current lengths. After a DESCRIBE, the array contains the number of characters in each indicator-variable name.

Converting Data

This section provides more detail about the T (datatype) descriptor array. In host programs that use neither datatype equivalencing nor dynamic SQL Method 4, the conversion between Oracle internal and external datatypes is determined at precompile time. By default, the precompiler assigns a specific external datatype to each host variable in the Declare Section. For example, the precompiler assigns the INTEGER external datatype to host variables of type int.

However, Method 4 lets you control data conversion and formatting. You specify conversions by setting datatype codes in the T descriptor array.

Coercing Datatypes

For a select descriptor, DESCRIBE SELECT LIST can return any of the Oracle internal datatypes. Often, as in the case of character data, the internal datatype corresponds exactly to the external datatype you want to use. However, a few internal datatypes map to external datatypes that can be difficult to handle. So, you might want to reset some elements in the T descriptor array. For example, you might want to reset NUMBER values to FLOAT values, which correspond to float values in C. Oracle does any necessary conversion between internal and external datatypes at FETCH time. So, be sure to reset the datatypes after the DESCRIBE SELECT LIST but before the FETCH.

For a bind descriptor, DESCRIBE BIND VARIABLES does not return the datatypes of bind variables, only their number and names. Therefore, you must explicitly set the T array of datatype codes to tell Oracle the external datatype of each bind variable. Oracle does any necessary conversion between external and internal datatypes at OPEN time.

When you reset datatype codes in the T descriptor array, you are "coercing datatypes." For example, to coerce the ith select-list value to STRING, you use the following statement:

/* Coerce select-list value to STRING. */ 
select_des->T[i] = 5; 

When coercing a NUMBER select-list value to STRING for display purposes, you must also extract the precision and scale bytes of the value and use them to compute a maximum display length. Then, before the FETCH, you must reset the appropriate element of the L (length) descriptor array to tell Oracle the buffer length to use.

For example, if DESCRIBE SELECT LIST finds that the ith select-list item is of type NUMBER, and you want to store the returned value in a C variable declared as float, simply set T[i] to 4 and L[i] to the length of floats on your system.

SQLNumberPrecV6(dvoid *runtime_context, int *length, int *precision,
                int *scale); 

/* Declare variables for the function call. */ 
sqlda         *select_des;  /* pointer to select descriptor */ 
int            prec;        /* precision                    */ 
int            scal;        /* scale                        */ 
extern void SQLNumberPrecV6();  /* Declare library function. */ 
/* Extract precision and scale. */ 
SQLNumberPrecV6(SQL_SINGLE_RCTX, &(select_des->L[i]), &prec, &scal); 
/* Allow for maximum size of NUMBER. */ 
if (prec == 0) 
    prec = 38; 
/* Allow for possible decimal point and sign. */ 
select_des->L[i] = prec + 2; 
/* Allow for negative scale. */ 
if (scal < 0) 
    select_des->L[i] += -scal; 

Handling NULL/Not NULL Datatypes

For every select-list column (not expression), DESCRIBE SELECT LIST returns a NULL/not NULL indication in the datatype array T of the select descriptor. If the ith select-list column is constrained to be not NULL, the high-order bit of T[i] is clear; otherwise, it is set.

Before using the datatype in an OPEN or FETCH statement, if the NULL/not NULL bit is set, you must clear it. (Never set the bit.)

You can use the library function SQLColumnNullCheck() (previously was called sqlnul()) to find out if a column allows NULLs, and to clear the datatype's NULL/not NULL bit. You call SQLColumnNullCheck() using the syntax:

SQLColumnNullCheck(dvoid *context, unsigned short *value_type, 
      unsigned short *type_code, int *null_status);

where:

The following example shows how to use SQLColumnNullCheck():

/* Declare variables for the function call. */ 
sqlda  *select_des;      /* pointer to select descriptor */ 
unsigned short   dtype;  /* datatype without null bit    */ 
int   nullok;            /* 1 = null, 0 = not null       */ 
extern void SQLColumnNullCheck();    /* Declare library function.    */ 
/* Find out whether column is not null. */ 
SQLColumnNUllCheck(SQL_SINGLE_RCTX, (unsigned short *)&(select_des->T[i]), &dtype, &nullok); 
if (nullok) 
{ 
    /* Nulls are allowed. */ 
    ... 
    /* Clear the null/not null bit. */ 
SQLColumnNullCheck(SQL_SINGLE_RCTX, &(select_des->T[i]), &(select_des->T[i]), &nullok); 
} 

Notice that the first and second arguments in the second call to the SQLColumnNullCheck() function point to the ith element in the array of datatypes, and that all three parameters are addresses.

Declare a Host String

Your program needs a host variable to store the text of the dynamic SQL statement. The host variable (select_stmt in our example) must be declared as a character string.

... 
    int      emp_number; 
    VARCHAR  emp_name[10]; 
    VARCHAR  select_stmt[120]; 
    float    bonus; 

Declare the SQLDAs

In our example, instead of hardcoding the SQLDA data structure, you use INCLUDE to copy it into your program, as follows:

#include <sqlda.h>

Then, because the query might contain an unknown number of select-list items or placeholders for bind variables, you declare pointers to select and bind descriptors, as follows:

sqlda  *select_des; 
sqlda  *bind_des; 

Allocate Storage Space for the Descriptors

Recall that you allocate storage space for a descriptor with the SQLSQLDAAlloc() library function. The syntax, using ANSI C notation, is:

SQLDA *SQLSQLDAAlloc(dvoid *context, unsigned int max_vars, unsigned int
max_name, unsigned int max_ind_name);

The SQLSQLDAAlloc() function allocates the descriptor structure and the arrays addressed by the pointer variables V, L, T, and I.

If max_name is nonzero, arrays addressed by the pointer variables S, M, and C are allocated. If max_ind_name is nonzero, arrays addressed by the pointer variables X, Y, and Z are allocated. No space is allocated if max_name and max_ind_name are zero.

If SQLSQLDAAlloc() succeeds, it returns a pointer to the structure. If SQLSQLDAAlloc() fails, it returns a zero.

In our example, you allocate select and bind descriptors, as follows:

select_des = SQLSQLDAAlloc(SQL_SINGLE_RCTX, 3, (size_t) 5, (size_t) 0); 
bind_des = SQLSQLDAAlloc(SQL_SINGLE_RCTX, 3, (size_t) 5, (size_t) 4); 

For select descriptors, always set max_ind_name to zero so that no space is allocated for the array addressed by X.

Set the Maximum Number to DESCRIBE

Next, you set the maximum number of select-list items or placeholders that can be DESCRIBEd, as follows:

select_des->N = 3; 
bind_des->N = 3; 

Figure 15-2 and Figure 15-3 represent the resulting descriptors.

Figure 15-2 Initialized Select Descriptor

Description of the illustration lnpcc011.gif

Figure 15-3 Initialized Bind Descriptor

Description of the illustration lnpcc012.gif

Put the Query Text in the Host String

Continuing our example, you prompt the user for a SQL statement, then store the input string in select_stmt, as follows:

printf("\n\nEnter SQL statement: "); 
gets(select_stmt.arr); 
select_stmt.len = strlen(select_stmt.arr); 

We assume the user entered the following string:

"SELECT ename, empno, comm FROM emp WHERE comm < :bonus" 

PREPARE the Query from the Host String

PREPARE parses the SQL statement and gives it a name. In our example, PREPARE parses the host string select_stmt and gives it the name sql_stmt, as follows:

EXEC SQL PREPARE sql_stmt FROM :select_stmt; 

DECLARE a Cursor

DECLARE CURSOR defines a cursor by giving it a name and associating it with a specific SELECT statement.

To declare a cursor for static queries, you use the following syntax:

EXEC SQL DECLARE cursor_name CURSOR FOR SELECT ... 

To declare a cursor for dynamic queries, the statement name given to the dynamic query by PREPARE is substituted for the static query. In our example, DECLARE CURSOR defines a cursor named emp_cursor and associates it with sql_stmt, as follows:

EXEC SQL DECLARE emp_cursor CURSOR FOR sql_stmt; 

DESCRIBE the Bind Variables

DESCRIBE BIND VARIABLES puts descriptions of placeholders into a bind descriptor. In our example, DESCRIBE readies bind_des, as follows:

EXEC SQL DESCRIBE BIND VARIABLES FOR sql_stmt INTO bind_des; 

Note that bind_des must not be prefixed with a colon.

The DESCRIBE BIND VARIABLES statement must follow the PREPARE statement but precede the OPEN statement.

Figure 15-4 shows the bind descriptor in our example after the DESCRIBE. Notice that DESCRIBE has set F to the actual number of placeholders found in the processed SQL statement.

Figure 15-4 Bind Descriptor after the DESCRIBE

Description of the illustration lnpcc013.gif

Reset Number of Placeholders

Next, you must reset the maximum number of placeholders to the number actually found by DESCRIBE, as follows:

bind_des->N = bind_des->F; 

Get Values and Allocate Storage for Bind Variables

Your program must get values for the bind variables found in the SQL statement, and allocate memory for them. How the program gets the values is up to you. For example, they can be hardcoded, read from a file, or entered interactively.

In our example, a value must be assigned to the bind variable that replaces the placeholder bonus in the query WHERE clause. So, you choose to prompt the user for the value, then process it as follows:

for (i = 0; i < bind_des->F; i++) 
{ 
    printf("\nEnter value of bind variable %.*s:\n? ", 
        (int) bind_des->C[i], bind_des->S[i]); 
    gets(hostval); 
    /* Set length of value. */ 
    bind_des->L[i] = strlen(hostval); 
    /* Allocate storage for value and null terminator. */ 
    bind_des->V[i] = malloc(bind_des->L[i] + 1); 
    /* Allocate storage for indicator value. */ 
    bind_des->I[i] = (unsigned short *) malloc(sizeof(short)); 
    /* Store value in bind descriptor. */ 
    strcpy(bind_des->V[i], hostval); 
    /* Set value of indicator variable. */ 
    *(bind_des->I[i]) = 0;  /* or -1 if "null" is the value */ 
    /* Set datatype to STRING. */ 
    bind_des->T[i] = 5; 
} 

Assuming that the user supplied a value of 625 for bonus, Figure 15-5 shows the resulting bind descriptor. Notice that the value is null-terminated.

Figure 15-5 Bind Descriptor after Assigning Values

Description of the illustration lnpcc014.gif

OPEN the Cursor

The OPEN statement used for dynamic queries is like that used for static queries except that the cursor is associated with a bind descriptor. Values determined at run time and stored in buffers addressed by elements of the bind descriptor arrays are used to evaluate the SQL statement. With queries, the values are also used to identify the active set.

In our example, OPEN associates emp_cursor with bind_des, as follows:

EXEC SQL OPEN emp_cursor USING DESCRIPTOR bind_des; 

Remember, bind_des must not be prefixed with a colon.

Then, OPEN executes the SQL statement. With queries, OPEN also identifies the active set and positions the cursor at the first row.

DESCRIBE the Select List

If the dynamic SQL statement is a query, the DESCRIBE SELECT LIST statement must follow the OPEN statement but precede the FETCH statement.

DESCRIBE SELECT LIST puts descriptions of select-list items in a select descriptor. In our example, DESCRIBE readies select_des, as follows:

EXEC SQL DESCRIBE SELECT LIST FOR sql_stmt INTO select_des;
 

Accessing the Oracle data dictionary, DESCRIBE sets the length and datatype of each select-list value.

Figure 15-6 shows the select descriptor in our example after the DESCRIBE. Notice that DESCRIBE has set F to the actual number of items found in the query select list. If the SQL statement is not a query, F is set to zero.

Also notice that the NUMBER lengths are not usable yet. For columns defined as NUMBER, you must use the library function SQLNumberPrecV6() to extract precision and scale.

Figure 15-6 Select Descriptor after the DESCRIBE

Description of the illustration lnpcc015.gif

Reset Number of Select-List Items

Next, you must reset the maximum number of select-list items to the number actually found by DESCRIBE, as follows:

select_des->N = select_des->F; 

Reset Length/Datatype of Each Select-list Item

In our example, before FETCHing the select-list values, you allocate storage space for them using the library function malloc(). You also reset some elements in the length and datatype arrays for display purposes.

for (i=0; i<select_des->F; i++) 
{ 
    /* Clear null bit. */ 
    SQLColumnNullCheck(SQL_SINGLE_RCTX, (unsigned short *)&(select_des->T[i]),
         (unsigned short *)&(select_des->T[i]), &nullok); 
    /* Reset length if necessary. */ 
    switch(select_des->T[i]) 
    { 
        case  1: break; 
        case  2: SQLNumberPrecV6(SQL_SINGLE_RCTX, (unsigned long *)
                      &(select_des->L[i]), &prec, &scal); 
                 if (prec == 0) prec = 40; 
                 select_des->L[i] = prec + 2; 
                 if (scal < 0) select_des->L[i] += -scal; 
                 break; 
        case  8: select_des->L[i] = 240; 
                 break; 
        case 11: select_des->L[i] = 18; 
                 break; 
        case 12: select_des->L[i] = 9; 
                 break; 
        case 23: break; 
        case 24: select_des->L[i] = 240; 
                 break; 
    } 
    /* Allocate storage for select-list value. */ 
    select_des->V[i] = malloc(select_des->L[i]+1); 
    /* Allocate storage for indicator value. */ 
    select_des->I[i] = (short *)malloc(sizeof(short *)); 
    /* Coerce all datatypes except LONG RAW to STRING. */ 
    if (select_des->T[i] != 24) select_des->T[i] = 5; 
} 

Figure 15-7 shows the resulting select descriptor. Notice that the NUMBER lengths are now usable and that all the datatypes are STRING. The lengths in L[1] and L[2] are 6 and 9 because we increased the DESCRIBEd lengths of 4 and 7 by 2 to allow for a possible sign and decimal point.

Figure 15-7 Select Descriptor before the FETCH

Description of the illustration lnpcc016.gif

FETCH Rows from the Active Set

FETCH returns a row from the active set, stores select-list values in the data buffers, and advances the cursor to the next row in the active set. If there are no more rows, FETCH sets sqlca.sqlcode to the "no data found" Oracle error code. In our example, FETCH returns the values of columns ENAME, EMPNO, and COMM to select_des, as follows:

EXEC SQL FETCH emp_cursor USING DESCRIPTOR select_des;
 

Figure 15-8 shows the select descriptor in our example after the FETCH. Notice that Oracle has stored the select-list and indicator values in the data buffers addressed by the elements of V and I.

For output buffers of datatype 1, Oracle, using the lengths stored in the L array, left-justifies CHAR or VARCHAR2 data and right-justifies NUMBER data. For output buffer of type 5 (STRING), Oracle left-justifies and null terminates CHAR, VARCHAR2, and NUMBER data.

The value 'MARTIN' was retrieved from a VARCHAR2(10) column in the EMP table. Using the length in L[0], Oracle left-justifies the value in a 10-byte field, filling the buffer.

The value 7654 was retrieved from a NUMBER(4) column and coerced to '7654'. However, the length in L[1] was increased by 2 to allow for a possible sign and decimal point. So, Oracle left-justifies and null terminates the value in a 6-byte field.

The value 482.50 was retrieved from a NUMBER(7,2) column and coerced to '482.50'. Again, the length in L[2] was increased by 2. So, Oracle left-justifies and null terminates the value in a 9-byte field.

Get and Process Select-List Values

After the FETCH, your program can process the returned values. In our example, values for columns ENAME, EMPNO, and COMM are processed.

Figure 15-8 Selected Descriptor after the FETCH

Description of the illustration lnpcc017.gif

Deallocate Storage

You use the free() library function to deallocate the storage space allocated by malloc(). The syntax is as follows:

free(char *pointer); 

In our example, you deallocate storage space for the values of the select-list items, bind variables, and indicator variables, as follows:

for (i = 0; i < select_des->F; i++)   /* for select descriptor */ 
{ 
    free(select_des->V[i]); 
    free(select_des->I[i]); 
} 
for (i = 0; i < bind_des->F; i++)   /* for bind descriptor */ 
{ 
    free(bind_des->V[i]); 
    free(bind_des->I[i]); 
} 

You deallocate storage space for the descriptors themselves with the SQLSQLDAFree() library function, using the following syntax:

SQLSQLDAFree(context, descriptor_name); 

The descriptor must have been allocated using SQLSQLDAAlloc(). Otherwise, the results are unpredictable.

In our example, you deallocate storage space for the select and bind descriptors as follows:

SQLSQLDAFree(SQL_SINGLE_RCTX, select_des); 
SQLSQLDAFree(SQL_SINGLE_RCTX, bind_des); 

CLOSE the Cursor

CLOSE disables the cursor. In our example, CLOSE disables emp_cursor as follows:

EXEC SQL CLOSE emp_cursor; 

Using Host Arrays

To use input or output host arrays with Method 4, you must use the optional FOR clause to tell Oracle the size of your host array.

You must set descriptor entries for the ith select-list item or bind variable using the syntax

V[i] = array_address; 
L[i] = element_size; 

where array_address is the address of the host array, and element_size is the size of one array element.

Then, you must use a FOR clause in the EXECUTE or FETCH statement (whichever is appropriate) to tell Oracle the number of array elements you want to process. This procedure is necessary because Oracle has no other way of knowing the size of your host array.

In the complete program example later, three input host arrays are used to INSERT rows into the EMP table. EXECUTE can be used for Data Manipulation Language statements other than queries with Method 4.

#include <stdio.h>
#include <sqlcpr.h>
#include <sqlda.h>
#include <sqlca.h>
 
#define NAME_SIZE    10
#define INAME_SIZE   10
#define ARRAY_SIZE    5
 
/* connect string */
char *username = "scott/tiger";
 
char *sql_stmt =
"INSERT INTO emp (empno, ename, deptno) VALUES (:e, :n, :d)";
int  array_size = ARRAY_SIZE;  /* must have a host variable too */
 
SQLDA   *binda;
 
char    names[ARRAY_SIZE][NAME_SIZE];
int     numbers[ARRAY_SIZE], depts[ARRAY_SIZE];

/* Declare and initialize indicator vars. for empno and deptno columns */
short   ind_empno[ARRAY_SIZE] = {0,0,0,0,0};
short   ind_dept[ARRAY_SIZE] = {0,0,0,0,0};
 
main() 
{ 
    EXEC SQL WHENEVER SQLERROR GOTO sql_error; 
 
/* Connect */ 
    EXEC SQL CONNECT :username; 
    printf("Connected.\n"); 
 
/* Allocate the descriptors and set the N component. 
   This must be done before the DESCRIBE. */ 
    binda = SQLSQLDAAlloc(SQL_SINGLE_RCTX, 3, NAME_SIZE, INAME_SIZE); 
    binda->N = 3; 
 
/* Prepare and describe the SQL statement. */ 
    EXEC SQL PREPARE stmt FROM :sql_stmt; 
    EXEC SQL DESCRIBE BIND VARIABLES FOR stmt INTO binda; 
 
/* Initialize the descriptors. */ 
    binda->V[0] = (char *) numbers; 
    binda->L[0] = (long) sizeof (int); 
    binda->T[0] = 3; 
    binda->I[0] = ind_empno; 
 
    binda->V[1] = (char *) names; 
    binda->L[1] = (long) NAME_SIZE; 
    binda->T[1] = 1; 
    binda->I[1] = (short *)0;
 
    binda->V[2] = (char *) depts; 
    binda->L[2] = (long) sizeof (int); 
    binda->T[2] = 3; 
    binda->I[2] = ind_dept; 
 
/* Initialize the data buffers. */ 
    strcpy(&names[0] [0], "ALLISON"); 
    numbers[0] = 1014; 
    depts[0] = 30; 
 
    strcpy(&names[1] [0], "TRUSDALE"); 
    numbers[1] = 1015; 
    depts[1] = 30; 
 
    strcpy(&names[2] [0], "FRAZIER"); 
    numbers[2] = 1016; 
    depts[2] = 30; 
 
    strcpy(&names[3] [0], "CARUSO"); 
    numbers[3] = 1017; 
    ind_dept[3] = -1;       /* set indicator to -1 to insert NULL */
    depts[3] = 30;          /* value in depts[3] is ignored */ 
 
    strcpy(&names[4] [0], "WESTON"); 
    numbers[4] = 1018;
    depts[4] = 30;
 
/* Do the INSERT. */
    printf("Adding to the Sales force...\n");
    
    EXEC SQL FOR :array_size
    EXECUTE stmt USING DESCRIPTOR binda;
 
/*  Print rows-processed count. */
    printf("%d rows inserted.\n\n", sqlca.sqlerrd[2]);
    EXEC SQL COMMIT RELEASE;
    exit(0);
 
sql_error: 
/* Print Oracle error message. */
    printf("\n%.70s", sqlca.sqlerrm.sqlerrmc);
    EXEC SQL WHENEVER SQLERROR CONTINUE; 
    EXEC SQL ROLLBACK RELEASE; 
    exit(1); 
} 

sample12.pc

A simple dynamic SQL example using array fetches is shown in file sample12.pc in the demo directory.