PREPARE Statements

PREPARE, EXECUTE and DEALLOCATE PREPARE Statements

PREPARE prepares a statement for execution

EXECUTE executes a prepared statement

DEALLOCATE PREPARE releases a prepared statement

SET @s = 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
PREPARE stmt2 FROM @s;
SET @a = 6;
SET @b = 8;
EXECUTE stmt2 USING @a, @b;

Result:

+------------+
| hypotenuse |
+------------+
|         10 |
+------------+

Finally,

DEALLOCATE PREPARE stmt2;

Notes:

• You must use @variables, not DECLAREd variables for FROM @s
• A primary use for Prepare, etc, is to ‘construct’ a query for situations where binding will not work, such as inserting the table name.

Alter table with add column

SET v_column_definition := CONCAT(
  v_column_name
  ,' ',v_column_type
  ,' ',v_column_options
);

SET @stmt := CONCAT('ALTER TABLE ADD COLUMN ', v_column_definition);

PREPARE stmt FROM @stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

Construct and execute

(This is a request for a good example that shows how to construct a SELECT using CONCAT, then prepare+execute it. Please emphasize the use of @variables versus DECLAREd variables — it makes a big difference, and it is something that novices (include myself) stumble over.)

Syntax

• PREPARE stmt_name FROM preparable_stmt
• EXECUTE stmt_name [USING @var_name [, @var_name] …]
• {DEALLOCATE | DROP} PREPARE stmt_name