Using descriptors v16

Dynamic SQL allows a client application to execute SQL statements that are composed at runtime. This ability is useful when you don't know the content or form for a statement when you're writing a client application. ECPGPlus doesn't allow you to use a host variable in place of an identifier (such as a table name, column name, or index name). Instead, use dynamic SQL statements to build a string that includes the information, and then execute that string. The string is passed between the client and the server in the form of a descriptor. A descriptor is a data structure that contains both the data and the information about the shape of the data.

Overview of the client application flow

A client application must use a GET DESCRIPTOR statement to retrieve information from a descriptor. The basic flow of a client application using dynamic SQL is:

1. Use an ALLOCATE DESCRIPTOR statement to allocate a descriptor for the result set (select list).
2. Use an ALLOCATE DESCRIPTOR statement to allocate a descriptor for the input parameters (bind variables).
3. Obtain, assemble, or compute the text of an SQL statement.
4. Use a PREPARE statement to parse and check the syntax of the SQL statement.
5. Use a DESCRIBE statement to describe the select list into the select-list descriptor.
6. Use a DESCRIBE statement to describe the input parameters into the bind-variables descriptor.
7. Prompt the user (if required) for a value for each input parameter. Use a SET DESCRIPTOR statement to assign the values into a descriptor.
8. Use a DECLARE CURSOR statement to define a cursor for the statement.
9. Use an OPEN CURSOR statement to open a cursor for the statement.
10. Use a FETCH statement to fetch each row from the cursor, storing each row in select-list descriptor.
11. Use a GET DESCRIPTOR command to interrogate the select-list descriptor to find the value of each column in the current row.
12. Use a CLOSE CURSOR statement to close the cursor and free any cursor resources.

Descriptor attributes

A descriptor can contain these attributes.

Example: Using a descriptor to return data

The following simple application executes an SQL statement entered by an end user. The code sample shows:

• How to use a SQL descriptor to execute a SELECT statement.
• How to find the data and metadata returned by the statement.

The application accepts an SQL statement from an end user, tests the statement to see if it includes the SELECT keyword, and executes the statement.

Using a SQL descriptor to execute a SELECT statement

When invoking the application, an end user must provide the name of the database on which to perform the SQL statement and a string that contains the text of the query.

For example, a user might invoke the sample with the following command:

./exec_stmt edb "SELECT * FROM emp"


/************************************************************
/* exec_stmt.pgc
*
*/

#include <stdio.h>
#include <stdlib.h>
#include <sql3types.h>
#include <sqlca.h>

EXEC SQL WHENEVER SQLERROR SQLPRINT;
static void print_meta_data( char * desc_name );

char *md1 = "col field                data              ret";
char *md2 = "num name                 type              len";
char *md3 = "--- -------------------- ----------------- ---";

int main( int argc, char *argv[] )
{

  EXEC SQL BEGIN DECLARE SECTION;
    char  *db   = argv[1];
    char  *stmt = argv[2];
    int    col_count;
  EXEC SQL END DECLARE SECTION;

  EXEC SQL CONNECT TO :db;

  EXEC SQL ALLOCATE DESCRIPTOR parse_desc;
  EXEC SQL PREPARE query FROM :stmt;
  EXEC SQL DESCRIBE query INTO SQL DESCRIPTOR parse_desc;
  EXEC SQL GET DESCRIPTOR 'parse_desc' :col_count = COUNT;

if( col_count == 0 )
{
    EXEC SQL EXECUTE IMMEDIATE :stmt;

    if( sqlca.sqlcode >= 0 )
      EXEC SQL COMMIT;
}
else
{
    int row;

    EXEC SQL ALLOCATE DESCRIPTOR row_desc;
    EXEC SQL DECLARE my_cursor CURSOR FOR query;
    EXEC SQL OPEN my_cursor;

    for( row = 0; ; row++ )
    {
       EXEC SQL BEGIN DECLARE SECTION;
         int     col;
       EXEC SQL END DECLARE SECTION;
       EXEC SQL FETCH IN my_cursor
         INTO SQL DESCRIPTOR row_desc;

       if( sqlca.sqlcode != 0 )
          break;

       if( row == 0 )
          print_meta_data( "row_desc" );

       printf("[RECORD %d]\n", row+1);

       for( col = 1; col <= col_count; col++ )
       {
           EXEC SQL BEGIN DECLARE SECTION;
             short   ind;
             varchar val[40+1];
             varchar name[20+1];
           EXEC SQL END DECLARE SECTION;

           EXEC SQL GET DESCRIPTOR 'row_desc'
             VALUE :col
             :val = DATA, :ind = INDICATOR, :name = NAME;

           if( ind == -1 )
             printf( "  %-20s : <null>\n", name.arr );
           else if( ind > 0 )
             printf( "  %-20s : <truncated>\n", name.arr );
           else
             printf( "  %-20s : %s\n", name.arr, val.arr );
       }

       printf( "\n" );

    }
    printf( "%d rows\n", row );
}

exit( 0 );
}

static void print_meta_data( char *desc_name )
{
   EXEC SQL BEGIN DECLARE SECTION;
     char  *desc = desc_name;
     int    col_count;
     int    col;
   EXEC SQL END DECLARE SECTION;

static char *types[] =
{
  "unused           ",
  "CHARACTER        ",
  "NUMERIC          ",
  "DECIMAL          ",
  "INTEGER          ",
  "SMALLINT         ",
  "FLOAT            ",
  "REAL             ",
  "DOUBLE           ",
  "DATE_TIME        ",
  "INTERVAL         ",
  "unused           ",
  "CHARACTER_VARYING",
  "ENUMERATED       ",
  "BIT              ",
  "BIT_VARYING      ",
  "BOOLEAN          ",
  "abstract         "
};

EXEC SQL GET DESCRIPTOR :desc :col_count = count;


printf( "%s\n", md1 );
printf( "%s\n", md2 );
printf( "%s\n", md3 );

for( col = 1; col <= col_count; col++ )
{

   EXEC SQL BEGIN DECLARE SECTION;
     int     type;
     int     ret_len;
     varchar name[21];
   EXEC SQL END DECLARE SECTION;
   char *type_name;

   EXEC SQL GET DESCRIPTOR :desc
     VALUE :col
     :name = NAME,
     :type = TYPE,
     :ret_len = RETURNED_OCTET_LENGTH;

   if( type > 0 && type < SQL3_abstract )
     type_name = types[type];
   else
     type_name = "unknown";

   printf( "%02d: %-20s %-17s %04d\n",
     col, name.arr, type_name, ret_len );
}
printf( "\n" );
}

/************************************************************

The code sample begins by including the prototypes and type definitions for the C stdio and stdlib libraries, SQL data type symbols, and the SQLCA (SQL communications area) structure:

#include <stdio.h>
#include <stdlib.h>
#include <sql3types.h>
#include <sqlca.h>

The sample provides minimal error handling. When the application encounters a SQL error, it prints the error message to screen:

EXEC SQL WHENEVER SQLERROR SQLPRINT;

Finding the data and metadata returned by the statement

The application includes a forward-declaration for a function named print_meta_data() that prints the metadata found in a descriptor:

static void print_meta_data( char * desc_name );

The following code specifies the column header information that the application uses when printing the metadata:

char *md1 = "col field data ret";
char *md2 = "num name type len";
char *md3 = "--- -------------------- ----------------- ---";

int main( int argc, char *argv[] )
{

The following declaration section identifies the host variables to contain the name of the database the application connects to, the content of the SQL statement, and a host variable for the number of columns in the result set (if any).

EXEC SQL BEGIN DECLARE SECTION;
  char *db = argv[1];
  char *stmt = argv[2];
  int col_count;
EXEC SQL END DECLARE SECTION;

The application connects to the database using the default credentials:

EXEC SQL CONNECT TO :db;

Next, the application allocates a SQL descriptor to hold the metadata for a statement:

EXEC SQL ALLOCATE DESCRIPTOR parse_desc;

The application uses a PREPARE statement to check the syntax of the string provided by the user:

EXEC SQL PREPARE query FROM :stmt;

It also uses a DESCRIBE statement to move the metadata for the query into the SQL descriptor.

EXEC SQL DESCRIBE query INTO SQL DESCRIPTOR parse_desc;

Then, the application interrogates the descriptor to discover the number of columns in the result set and stores that in the host variable col_count.

EXEC SQL GET DESCRIPTOR parse_desc :col_count = COUNT;

If the column count is zero, the end user didn't enter a SELECT statement. The application uses an EXECUTE IMMEDIATE statement to process the contents of the statement:

if( col_count == 0 )
{
   EXEC SQL EXECUTE IMMEDIATE :stmt;

If the statement executes successfully, the application performs a COMMIT:

if( sqlca.sqlcode >= 0 )
  EXEC SQL COMMIT;
}
else
{

If the statement entered by the user is a SELECT statement (which we know because the column count is non-zero), the application declares a variable named row:

int row;

Then, the application allocates another descriptor that holds the description and the values of a specific row in the result set:

EXEC SQL ALLOCATE DESCRIPTOR row_desc;

The application declares and opens a cursor for the prepared statement:

EXEC SQL DECLARE my_cursor CURSOR FOR query;
EXEC SQL OPEN my_cursor;

It loops through the rows in the result set:

for( row = 0; ; row++ )
{
  EXEC SQL BEGIN DECLARE SECTION;
    int col;
  EXEC SQL END DECLARE SECTION;

Then, it uses a FETCH to retrieve the next row from the cursor into the descriptor:

EXEC SQL FETCH IN my_cursor INTO SQL DESCRIPTOR row_desc;

The application confirms that the FETCH didn't fail. If the FETCH fails, the application reached the end of the result set and breaks the loop:

if( sqlca.sqlcode != 0 )
   break;

The application checks to see if this is the first row of the cursor. If it is, the application prints the metadata for the row:

if( row == 0 )
   print_meta_data( "row_desc" );

Next, it prints a record header containing the row number:

printf("[RECORD %d]\n", row+1);

Then, it loops through each column in the row:

for( col = 1; col <= col_count; col++ )
{
   EXEC SQL BEGIN DECLARE SECTION;
     short ind;
     varchar val[40+1];
     varchar name[20+1];
   EXEC SQL END DECLARE SECTION;

The application interrogates the row descriptor (row_desc) to copy the column value :val, null indicator :ind, and column name :name into the host variables declared earlier. You can retrieve multiple items from a descriptor using a comma-separated list:

EXEC SQL GET DESCRIPTOR row_desc
  VALUE :col
  :val = DATA, :ind = INDICATOR, :name = NAME;

If the null indicator (ind) is negative, the column value is NULL. If the null indicator is greater than 0, the column value is too long to fit into the val host variable, so we print <truncated>. Otherwise, the null indicator is 0, meaning NOT NULL, so we print the value. In each case, we prefix the value (or <null> or <truncated>) with the name of the column.

if( ind == -1 )
  printf( " %-20s : <null>\n", name.arr );
else if( ind > 0 )
  printf( " %-20s : <truncated>\n", name.arr );
else
  printf( " %-20s : %s\n", name.arr, val.arr );
}

printf( "\n" );
}

When the loop terminates, the application prints the number of rows fetched and exits:

   printf( "%d rows\n", row );
  }

exit( 0 );
}

The print_meta_data() function extracts the metadata from a descriptor and prints the name, data type, and length of each column:

static void print_meta_data( char *desc_name )
{

The application declares host variables:

EXEC SQL BEGIN DECLARE SECTION;
  char *desc = desc_name;
  int col_count;
  int col;
EXEC SQL END DECLARE SECTION;

The application then defines an array of character strings that map data type values (numeric) into data type names. We use the numeric value found in the descriptor to index into this array. For example, if we find that a given column is of type 2, we can find the name of that type (NUMERIC) by writing types[2].

static char *types[] =
{
  "unused ",
  "CHARACTER ",
  "NUMERIC ",
  "DECIMAL ",
  "INTEGER ",
  "SMALLINT ",
  "FLOAT ",
  "REAL ",
  "DOUBLE ",
  "DATE_TIME ",
  "INTERVAL ",
  "unused ",
  "CHARACTER_VARYING",
  "ENUMERATED ",
  "BIT ",
  "BIT_VARYING ",
  "BOOLEAN ",
  "abstract "
};

The application retrieves the column count from the descriptor. The program refers to the descriptor using a host variable (desc) that contains the name of the descriptor. In most scenarios, you use an identifier to refer to a descriptor. In this case, the caller provided the descriptor name, so we can use a host variable to refer to the descriptor.

EXEC SQL GET DESCRIPTOR :desc :col_count = count;

The application prints the column headers defined at the beginning of this application:

printf( "%s\n", md1 );
printf( "%s\n", md2 );
printf( "%s\n", md3 );

Then, it loops through each column found in the descriptor and prints the name, type, and length of each column.

for( col = 1; col <= col_count; col++ )
{
  EXEC SQL BEGIN DECLARE SECTION;
    int type;
    int ret_len;
    varchar name[21];
  EXEC SQL END DECLARE SECTION;
  char *type_name;

It retrieves the name, type code, and length of the current column:

EXEC SQL GET DESCRIPTOR :desc
 VALUE :col
 :name = NAME,
 :type = TYPE,
 :ret_len = RETURNED_OCTET_LENGTH;

If the numeric type code matches a 'known' type code (that is, a type code found in the types[] array), it sets type_name to the name of the corresponding type. Otherwise, it sets type_name to "unknown":

if( type > 0 && type < SQL3_abstract )
  type_name = types[type];
else
  type_name = "unknown";

It then prints the column number, name, type name, and length:

   printf( "%02d: %-20s %-17s %04d\n",
     col, name.arr, type_name, ret_len );
  }
  printf( "\n" );
}

Invoke the sample application with the following command:

./exec_stmt test "SELECT * FROM emp WHERE empno IN(7902, 7934)"

The application returns:

col field                data              ret
num name                 type              len
--- -------------------- ----------------- ---
01: empno                NUMERIC           0004
02: ename                CHARACTER_VARYING 0004
03: job                  CHARACTER_VARYING 0007
04: mgr                  NUMERIC           0004
05: hiredate             DATE_TIME         0018
06: sal                  NUMERIC           0007
07: comm                 NUMERIC           0000
08: deptno               NUMERIC           0002

[RECORD 1]
  empno                : 7902
  ename                : FORD
  job                  : ANALYST
  mgr                  : 7566
  hiredate             : 03-DEC-81 00:00:00
  sal                  : 3000.00
  comm                 : <null>
  deptno               : 20

[RECORD 2]
  empno                : 7934
  ename                : MILLER
  job                  : CLERK
  mgr                  : 7782
  hiredate             : 23-JAN-82 00:00:00
  sal                  : 1300.00
  comm                 : <null>
  deptno               : 10

2 rows

• On this page
• Overview of the client application flow
• Descriptor attributes
• Example: Using a descriptor to return data