Loops in MySQL

In MySQL, loops let you repeat a sequence of statements until a certain condition is met. You can use them to repeat execution of code blocks in stored procedures, functions and triggers.

Loop Types in MySQL

MySQL supports three main types of loops: LOOP, REPEAT, and WHILE. Each serves a different purpose and is used based on the logic requirements.

LOOP Statement

The LOOP statement is the simplest form of loop in MySQL. It runs indefinitely until it encounters a LEAVE statement.

LOOP_NAME: LOOP
    -- Statements to be executed
    IF condition THEN
        LEAVE LOOP_NAME;
    END IF;
END LOOP LOOP_NAME;

REPEAT Statement

The REPEAT loop executes until a specified condition is true. It checks the condition after executing the loop’s statements.

REPEAT
    -- Statements to be executed
UNTIL condition
END REPEAT;

WHILE Statement

The WHILE loop executes as long as a specified condition is true, checking the condition before each iteration.

WHILE condition DO
    -- Statements to be executed
END WHILE;

Implementing Loops in Stored Procedures

Loops are commonly used in stored procedures. Here’s an example of using a WHILE loop within a stored procedure:

DELIMITER //
CREATE PROCEDURE LoopDemo()
BEGIN
    DECLARE v1 INT DEFAULT 5;

    WHILE v1 > 0 DO
        -- Perform actions
        SET v1 = v1 - 1;
    END WHILE;
END //
DELIMITER ;

Loop Control Statements

Control statements like LEAVE and ITERATE are used to control the flow of loops.

LEAVE Statement

LEAVE exits the loop when a certain condition is met. It is useful in LOOP and WHILE loops.

WHILE condition DO
    IF exit_condition THEN
        LEAVE loop_name;
    END IF;
    -- Loop statements
END WHILE loop_name;

ITERATE Statement

ITERATE skips the current loop iteration and starts the next one. It is particularly useful in LOOP and WHILE loops.

WHILE condition DO
    IF skip_condition THEN
        ITERATE loop_name;
    END IF;
    -- Loop statements
END WHILE loop_name;

Nested Loops

Nested loops, where one loop is placed inside another, are particularly useful for processing multi-dimensional data. Here’s an example:

DECLARE outer_counter INT DEFAULT 1;
DECLARE inner_counter INT DEFAULT 1;

WHILE outer_counter <= 5 DO
    WHILE inner_counter <= 3 DO
        -- Inner loop actions
        SET inner_counter = inner_counter + 1;
    END WHILE;
    SET inner_counter = 1;
    -- Outer loop actions
    SET outer_counter = outer_counter + 1;
END WHILE;

Common Use Cases for Loops

• Data Transformation: Manipulating or formatting a large set of data.
• Batch Processing: Executing operations on a batch of records.
• Recursive Operations: Handling hierarchical or tree-structured data.

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