Postgres Plus Advanced Server Oracle Compatibility Developer's Guide : 13.4 The SQLDA Structure

Previous PageTable Of ContentsNext Page

Postgres Plus Advanced Server Oracle Compatibility Developer's Guide

 

13.4 The SQLDA Structure

Oracle Dynamic SQL method 4 uses the SQLDA data structure to hold the data and metadata for a dynamic SQL statement. A SQLDA structure can describe a set of input parameters corresponding to the parameter markers found in the text of a dynamic statement or the result set of a dynamic statement. The layout of the SQLDA structure is:

struct SQLDA

{

int N; /* Number of entries */

char **V; /* Variables */

int *L; /* Variable lengths */

short *T; /* Variable types */

short **I; /* Indicators */

int F; /* Count of variables discovered by DESCRIBE */

char **S; /* Variable names */

short *M; /* Variable name maximum lengths */

short *C; /* Variable name actual lengths */

char **X; /* Indicator names */

short *Y; /* Indicator name maximum lengths */

short *Z; /* Indicator name actual lengths */

};

Parameters

N - maximum number of entries

The N structure member contains the maximum number of entries that the SQLDA may describe. This member is populated by the sqlald() function when you allocate the SQLDA structure. Before using a descriptor in an OPEN or FETCH statement, you must set N to the actual number of values described.

V - data values

The V structure member is a pointer to an array of data values.

For a SELECT-list descriptor, V points to an array of values returned by a FETCH statement (each member in the array corresponds to a column in the result set).

For a bind descriptor, V points to an array of parameter values (you must populate the values in this array before opening a cursor that uses the descriptor).

Your application must allocate the space required to hold each value.

L - length of each data value

The L structure member is a pointer to an array of lengths. Each member of this array must indicate the amount of memory available in the corresponding member of the V array. For example, if V[5] points to a buffer large enough to hold a 20-byte NULL-terminated string, L[5] should contain the value 21 (20 bytes for the characters in the string plus 1 byte for the NULL-terminator). Your application must set each member of the L array.

T - data types

The T structure member points to an array of data types, one for each column (or parameter) described by the descriptor.

For a bind descriptor, you must set each member of the T array to tell ECPGPlus the data type of each parameter.

For a SELECT-list descriptor, the DESCRIBE SELECT LIST statement sets each member of the T array to reflect the type of data found in the corresponding column.

You may change any member of the T array before executing a FETCH statement to force ECPGPlus to convert the corresponding value to a specific data type. For example, if the DESCRIBE SELECT LIST statement indicates that a given column is of type DATE, you may change the corresponding T member to request that the next FETCH statement return that value in the form of a NULL-terminated string. Each member of the T array is a numeric type code (see Section 13.3 for a list of type codes). The type codes returned by a DESCRIBE SELECT LIST statement differ from those expected by a FETCH statement. After executing a DESCRIBE SELECT LIST statement, each member of T encodes a data type and a flag indicating whether the corresponding column is nullable. You can use the sqlnul() function to extract the type code and nullable flag from a member of the T array. The signature of the sqlnul() function is as follows:

      void sqlnul(unsigned short *valType,
      unsigned short *typeCode,
      int *isNull)

For example, to find the type code and nullable flag for the third column of a descriptor named results, you would invoke sqlnul() as follows:

      sqlnul(&results->T[2], &typeCode, &isNull);

I - indicator variables

The I structure member points to an array of indicator variables. This array is allocated for you when your application calls the sqlald() function to allocate the descriptor.

For a SELECT-list descriptor, each member of the I array indicates whether the corresponding column contains a NULL (non-zero) or non-NULL (zero) value.

For a bind parameter, your application must set each member of the I array to indicate whether the corresponding parameter value is NULL.

F - number of entries

The F structure member indicates how many values are described by the descriptor (the N structure member indicates the maximum number of values which may be described by the descriptor; F indicates the actual number of values). The value of the F member is set by ECPGPlus when you execute a DESCRIBE statement. F may be positive, negative, or zero.

For a SELECT-list descriptor, F will contain a positive value if the number of columns in the result set is equal to or less than the maximum number of values permitted by the descriptor (as determined by the N structure member); 0 if the statement is not a SELECT statement, or a negative value if the query returns more columns than allowed by the N structure member.

For a bind descriptor, F will contain a positive number if the number of parameters found in the statement is less than or equal to the maximum number of values permitted by the descriptor (as determined by the N structure member); 0 if the statement contains no parameters markers, or a negative value if the statement contains more parameter markers than allowed by the N structure member.

If F contains a positive number (after executing a DESCRIBE statement), that number reflects the count of columns in the result set (for a SELECT-list descriptor) or the number of parameter markers found in the statement (for a bind descriptor). If F contains a negative value, you may compute the absolute value of F to discover how many values (or parameter markers) are required. For example, if F contains -24 after describing a SELECT list, you know that the query returns 24 columns.

S - column/parameter names

The S structure member points to an array of NULL-terminated strings.

For a SELECT-list descriptor, the DESCRIBE SELECT LIST statement sets each member of this array to the name of the corresponding column in the result set.

For a bind descriptor, the DESCRIBE BIND VARIABLES statement sets each member of this array to the name of the corresponding bind variable.

In this release, the name of each bind variable is determined by the left-to-right order of the parameter marker within the query - for example, the name of the first parameter is always ?0, the name of the second parameter is always ?1, and so on.

M - maximum column/parameter name length

The M structure member points to an array of lengths. Each member in this array specifies the maximum length of the corresponding member of the S array (that is, M[0] specifies the maximum length of the column/parameter name found at S[0]). This array is populated by the sqlald() function.

C - actual column/parameter name length

The C structure member points to an array of lengths. Each member in this array specifies the actual length of the corresponding member of the S array (that is, C[0] specifies the actual length of the column/parameter name found at S[0]).

This array is populated by the DESCRIBE statement.

X - indicator variable names

The X structure member points to an array of NULL-terminated strings - each string represents the name of a NULL indicator for the corresponding value.

This array is not used by ECPGPlus, but is provided for compatibility with Pro*C applications.

Y - maximum indicator name length

The Y structure member points to an array of lengths. Each member in this array specifies the maximum length of the corresponding member of the X array (that is, Y[0] specifies the maximum length of the indicator name found at X[0]).

This array is not used by ECPGPlus, but is provided for compatibility with Pro*C applications.

Z - actual indicator name length

The Z structure member points to an array of lengths. Each member in this array specifies the actual length of the corresponding member of the X array (that is, Z[0] specifies the actual length of the indicator name found at X[0]).

This array is not used by ECPGPlus, but is provided for compatibility with Pro*C applications.

Previous PageTable Of ContentsNext Page