再来一发！DB2 应用程序如何从数据库取数据 Fetch

The FETCH statement positions a cursor on the next row of its result table and assigns the values of that row to target variables.

Invocation

Although an interactive SQL facility might provide an interface that gives the appearance of interactive execution, this statement can only be embedded within an application program. It is an executable statement that cannot be dynamically prepared. When invoked using the command line processor, the syntax following cursor-name is optional and different from the SQL syntax.

For more information, refer to Using command line SQL statements and XQuery statements .

Authorization

For each global variable used as a cursor-variable-name or in the expression for an array-index, the privileges held by the authorization ID of the statement must include one of the following:

• READ privilege on the global variable that is not defined in a module
• EXECUTE privilege on the module of the global variable that is defined in a module

For each global variable used as an assignment-target, the privileges held by the authorization ID of the statement must include one of the following:

• WRITE privilege on the global variable that is not defined in a module
• EXECUTE privilege on the module of the global variable that is defined in a module

For the authorization required to use a cursor, see "DECLARE CURSOR".

Syntax

>>-FETCH--+------+--+-cursor-name----------+-------------------->
          '-FROM-'  '-cursor-variable-name-'   

           .-,---------------------.       
           V                       |       
>--+-INTO----| assignment-target |-+---+-----------------------><
   '-USING DESCRIPTOR--descriptor-name-'   

assignment-target

>>-+-global-variable-name-------------------+------------------><
   +-host-variable-name---------------------+   
   +-SQL-parameter-name---------------------+   
   +-SQL-variable-name----------------------+   
   +-transition-variable-name---------------+   
   +-array-variable-name--[--array-index--]-+   
   '-field-reference------------------------'   

Description

cursor-variable-name

Identifies the cursor to be used in the fetch operation. The cursor-variable-name must identify a cursor variable that is in scope. When the FETCH statement is executed, the underlying cursor of the cursor-variable-name must be in the open state. A FETCH statement using a cursor-variable-name can only be used within a compound SQL (compiled) statement.

INTO assignment-target

Identifies one or more targets for the assignment of output values. The first value in the result row is assigned to the first target in the list, the second value to the second target, and so on. Each assignment to an assignment-target is made in sequence through the list. If an error occurs on any assignment, the value is not assigned to the target, and no more values are assigned to targets. Any values that have already been assigned to targets remain assigned.

When the data type of every assignment-target is not a row type, then the value 'W' is assigned to the SQLWARN3 field of the SQLCA if the number of assignment-targets is less than the number of result column values.

If the data type of an assignment-target is a row type, then there must be exactly one assignment-target specified (SQLSTATE 428HR), the number of columns must match the number of fields in the row type, and the data types of the columns of the fetched row must be assignable to the corresponding fields of the row type (SQLSTATE 42821).

If the data type of an assignment-target is an array element, then there must be exactly one assignment-target specified.

global-variable-name

Identifies the global variable that is the assignment target.

host-variable-name

Identifies the host variable that is the assignment target. For LOB output values, the target can be a regular host variable (if it is large enough), a LOB locator variable, or a LOB file reference variable.

SQL-parameter-name

Identifies the parameter that is the assignment target.

SQL-variable-name

Identifies the SQL variable that is the assignment target. SQL variables must be declared before they are used.

transition-variable-name

Identifies the column to be updated in the transition row. A transition-variable-name must identify a column in the subject table of a trigger, optionally qualified by a correlation name that identifies the new value.

array-variable-name

Identifies an SQL variable, SQL parameter, or global variable of an array type.

[array-index]

An expression that specifies which element in the array will be the target of the assignment. For an ordinary array, the array-index expression must be assignable to INTEGER (SQLSTATE 428H1) and cannot be the null value. Its value must be between 1 and the maximum cardinality defined for the array (SQLSTATE 2202E). For an associative array, the array-index expression must be assignable to the index data type of the associative array (SQLSTATE 428H1) and cannot be the null value.

field-reference

Identifies the field within a row type value that is the assignment target. The field-reference must be specified as a qualified field-name where the qualifier identifies the row value in which the field is defined.

USING DESCRIPTOR descriptor-name

Identifies an SQLDA that must contain a valid description of zero or more host variables.

Before the FETCH statement is processed, the user must set the following fields in the SQLDA:

• SQLN to indicate the number of SQLVAR occurrences provided in the SQLDA.
• SQLDABC to indicate the number of bytes of storage allocated for the SQLDA.
• SQLD to indicate the number of variables used in the SQLDA when processing the statement.
• SQLVAR occurrences to indicate the attributes of the variables.

The SQLDA must have enough storage to contain all SQLVAR occurrences. Therefore, the value in SQLDABC must be greater than or equal to 16 + SQLN*(N), where N is the length of an SQLVAR occurrence.

If LOB or structured type result columns need to be accommodated, there must be two SQLVAR entries for every select-list item (or column of the result table).

SQLD must be set to a value greater than or equal to zero and less than or equal to SQLN.

The nth variable described in the SQLDA corresponds to the nth column of the result table of the cursor. The data type of each variable must be compatible with its corresponding column.

Each assignment to a variable is made according to specific rules. If the number of variables is less than the number of values in the row, the SQLWARN3 field of the SQLDA is set to 'W'. Note that there is no warning if there are more variables than the number of result columns. If an assignment error occurs, the value is not assigned to the variable, and no more values are assigned to variables. Any values that have already been assigned to variables remain assigned.

Notes

• Cursor position: An open cursor has three possible positions:
  • Before a row
  • On a row
  • After the last row.
  A cursor can only be on a row as a result of a FETCH statement. If the cursor is currently positioned on or after the last row of the result table:
  • SQLCODE is set to +100, and SQLSTATE is set to '02000'.
  • The cursor is positioned after the last row.
  • Values are not assigned to assignment targets.

  If the cursor is currently positioned before a row, it will be repositioned on that row, and values will be assigned to targets as specified by the INTO or USING clause.

  If the cursor is currently positioned on a row other than the last row, it will be repositioned on the next row and values of that row will be assigned to targets as specified by the INTO or USING clause.

  If a cursor is on a row, that row is called the current row of the cursor. A cursor referenced in an UPDATE or DELETE statement must be positioned on a row.

  It is possible for an error to occur that makes the state of the cursor unpredictable.

• When retrieving into LOB locators in situations where it is not necessary to retain the locator across FETCH statements, it is good practice to issue a FREE LOCATOR statement before issuing the next FETCH statement, as locator resources are limited.
• It is possible that a warning may not be returned on a FETCH. It is also possible that the returned warning applies to a previously fetched row. This occurs as a result of optimizations such as the use of system temporary tables or pushdown operators.
• Statement caching affects the behavior of an EXECUTE IMMEDIATE statement.
• DB2®CLI supports additional fetching capabilities. For instance, when a cursor's result table is read-only, the SQLFetchScroll() function can be used to position the cursor at any spot within that result table.
• For an updatable cursor, a lock is obtained on a row when it is fetched.
• If the cursor definition contains an SQL data change statement or invokes a routine that modifies SQL data, an error during the fetch operation does not cause the modified rows to be rolled back, even if the error results in the cursor being closed.

Examples

• Example 1:  In this C example, the FETCH statement fetches the results of the SELECT statement into the program variablesdnum,dname, andmnum. When no more rows remain to be fetched, the not found condition is returned.

     EXEC SQL DECLARE C1 CURSOR FOR
       SELECT DEPTNO, DEPTNAME, MGRNO FROM TDEPT
         WHERE ADMRDEPT = 'A00';
  
     EXEC SQL OPEN C1;
  
     while (SQLCODE==0) {
         EXEC SQL FETCH C1 INTO :dnum, :dname, :mnum;
     }
  
     EXEC SQL CLOSE C1;

• Example 2:  This FETCH statement uses an SQLDA.

     FETCH CURS USING DESCRIPTOR :sqlda3