MySQL - DEALLOCATE Statement

A prepared statement in MySQL represents a pre compiled statement. A statement is compiled and stored in a prepared statement and you can later execute this multiple times. Instead of values we pass place holders to this statement.

If you want to execute several identical queries (that differ by values only). You can use prepared statements. You can execute these statements in client libraries as well as in SQL scripts.

A SQL prepared statement is based on three statements namely −

  • PREPARE
  • EXECUTE
  • DEALLOCATE PREPARE

MySQL DEALLOCATE Statement

The DEALLOCATE PREPARE is used to de-allocate a PREPARED statement.

Syntax

Following is the syntax of the EXECUTE statement −

DEALLOCATE PREPARE stmt_name

Where stmt_name is the name of the prepared statement to be de-allocated.

Example

Suppose we have created a table named Employee in the database using the CREATE statement and inserted three records in it as shown below −

CREATE TABLE Employee(
   Name VARCHAR(255), 
   Salary INT, 
   Location VARCHAR(255)
);

You can prepare an INSERT statement with place holders instead of values as −

PREPARE prepared_stmt FROM 'INSERT INTO EMPLOYE VALUES (?, ?, ?)';
Statement prepared

Now execute the above created prepared statement −

EXECUTE prepared_stmt USING @name, @sal, @loc;

Following query de-allocates the above created prepared statement.

DEALLOCATE PREPARE prepared_stmt;

Verification

After de-allocating the prepared statement if you try to execute it again an error will be generated −

ERROR 1243 (HY000): Unknown prepared statement handler (prepared_stmt) 
given to EXECUTE

Example

We can also delete a prepared statement using this statement. Assume we have created another prepared statement as shown below −

--Preparing the statement
PREPARE prepared_stmt FROM 'SELECT * FROM EMPLOYE';
Statement prepared

--Executing the statement
EXECUTE prepared_stmt;

Output

Following is the output of the above program −

Name Salary Location
Amit 6554 Hyderabad
Sumith 5981 Vishakhapatnam
Sudha 7887 Vijayawada
Raghu 9878 Delhi

Following statement drops the above created prepared statement −

DROP PREPARE prepared_stmt;

Example

Assume we have created another table and populated it using the following queries −

Create table Student(
   Name Varchar(35), 
   age INT, 
   Score INT
);

Now, let's try to insert some records into the Student table −

INSERT INTO student values 
('Jeevan', 22, 8),
('Raghav', 26, –3),
('Khaleel', 21, –9),
('Deva', 30, 9);

You can choose the table to execute a query dynamically using this statement as shown below −

--Setting the table name dynamically
SET @table = 'Student';
SET @statement = CONCAT('SELECT * FROM ', @table);

--Preparing the statement
PREPARE prepared_stmt FROM @statement;
Statement prepared

--Executing the statement
EXECUTE prepared_stmt;

Output

The above mysql query will generate the following output −

Name age Score
22 8
Raghav 26 -3
Khaleel 21 -9
Deva 30 9 
--De-allocating the statement
DEALLOCATE PREPARE prepared_stmt;
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