PREPARED STATEMENT¶

In general, the prepared statement is executed through the interface functions of JDBC, PHP, or ODBC; it can also be executed in the SQL level. The following SQL statements are provided for execution of prepared statement.

• Prepare the SQL statement to execute.

  PREPARE stmt_name FROM preparable_stmt
  

• Execute the prepared statement.

  EXECUTE stmt_name [USING value [, value] ...]
  

• Drop the prepared statement.

  {DEALLOCATE | DROP} PREPARE stmt_name
  

PREPARE stmt_name FROM preparable_stmt

EXECUTE stmt_name [USING value [, value] ...]

PREPARE st FROM 'SELECT 1 + ?';
EXECUTE st USING 4;

   1+ ?:0
==========================
   5

PREPARE st FROM 'SELECT 1 + ?';
SET @a=3;
EXECUTE st USING @a;

   1+ ?:0
==========================
   4

PREPARE st FROM 'SELECT ? + ?';
EXECUTE st USING 1,3;

   ?:0 + ?:1
==========================
   4

PREPARE st FROM 'SELECT ? + ?';
EXECUTE st USING 'a','b';

   ?:0 + ?:1
==========================
   'ab'

PREPARE st FROM 'SELECT FLOOR(?)';
EXECUTE st USING '3.2';

   floor( ?:0 )
==========================
   3.000000000000000e+000

{DEALLOCATE | DROP} PREPARE stmt_name

DEALLOCATE PREPARE stmt1;

SET¶

The SET statement is the syntax that specifies user-defined variables and the method that you can use to store values.

You can create user-defined variables in two ways. One is to use the SET statement and the other is to use the assignment statement of user-defined variables within SQL statements. You can delete the user-defined variables that you defined with the DEALLOCATE or the DROP statements.

The user-defined variables are also called session variables as they are used for maintaining connections within one application. The user-defined variables are used within the part of a connection session, and the user-defined variables defined by an application cannot be accessed by other applications. When an application terminates connections, all variables will be removed automatically. The user-defined variables are limited to twenty per connection session for an application. If you already have twenty user-defined variables and want to define a new user-defined variable, you must remove some variables with the DROP VARIABLE statement.

You can use user-defined variables in most SQL statements. If you define user-defined variables and refer to them in one statement, the sequence is not guaranteed. That is, if you refer to the variables specified in the SELECT list of the HAVING, GROUP BY or ORDER BY clause, you may not get the values in the sequence you expect. You cannot also use user-defined variables as identifiers, such as column names or table names within SQL statements

The user-defined variables are not case-sensitive. The user-defined variable type can be one of the SHORT, INTEGER, BIGINT, FLOAT, DOUBLE, NUMERIC, CHAR, VARCHAR, BIT and BIT VARYING. Other types will be converted to the VARCHAR type.

SET @v1 = 1, @v2=CAST(1 AS BIGINT), @v3 = '123', @v4 = DATE'2010-01-01';

SELECT typeof(@v1), typeof(@v2), typeof(@v3), typeof(@v4);

   typeof(@v1)         typeof(@v2)         typeof(@v3)         typeof(@v4)
======================================================================================
  'integer'           'bigint'            'character (-1)'    'character varying (1073741823)

The user-defined variables can be changed when you define values.

SET @v = 'a';
SET @v1 = 10;

SELECT @v := 1, typeof(@v1), @v1:='1', typeof(@v1);

  @v := 1                typeof(@v1)          @v1 := '1'             typeof(@v1)
======================================================================================
  1                     'integer'             '1'                    'character (-1)'

<set_statement>
        : <set_statement>, <udf_assignment>
        | SET <udv_assignment>
        ;

<udv_assignment>
        : @<name> = <expression>
        | @<name> := <expression>
        ;

{DEALLOCATE|DROP} VARIABLE <variable_name_list>
<variable_name_list>
       : <variable_name_list> ',' @<name>

• You must define the variable names with alphanumeric characters and underscores (_).
• When you define the variables within SQL statements, you should use the ':=' operator.

The following example shows how to define the variable a and assign a value 1 to it.

SET @a = 1;
SELECT @a;

  @a
======================
  1

The following example shows how to count the number of rows in the SELECT statement by using the user-defined variable.

CREATE TABLE t (i INTEGER);
INSERT INTO t(i) VALUES(2),(4),(6),(8);

SET @a = 0;

SELECT @a := @a+1 AS row_no, i FROM t;

  row_no                          i
 ===================================
  1                               2
  2                               4
  3                               6
  4                               8

4 rows selected.

The following example shows how to use the user-defined variable as the input of bind parameter specified in the prepared statement.

SET @a:=3;

PREPARE stmt FROM 'SELECT i FROM t WHERE i < ?';
EXECUTE stmt USING @a;

            i
=============
            2

The following example shows how to declare the user-defined variable by using the ':=' operator.

SELECT @a := 1, @user_defined_variable := 'user defined variable';
UPDATE t SET i = (@var := 1);

The following example shows how to delete the user-defined variable a and user_defined_variable.

DEALLOCATE VARIABLE @a, @user_defined_variable;
DROP VARIABLE @a, @user_defined_variable;

DO¶

The DO statement executes the specified expression, but does not return the result. This can be used to determine whether or not the syntax of the expression is correct because an error is returned when a specified expression does not comply with the syntax. In general, the execution speed of the DO statement is higher than that of the SELECT statement because the database server does not return the operation result or errors.

DO expression

• expression : Specifies an expression.

DO 1+1;
DO SYSDATE + 1;
DO (SELECT count(*) FROM athlete);