mysql stored procedure loop

MySQL stored procedures are a type of precompiled code segments that can receive input parameters, execute SQL statements, and output results. Stored procedures are usually used to organize complex SQL operations, improve the performance of SQL queries, reduce the amount of data transmitted over the network, and implement transaction control and other functions. This article mainly introduces the loop control structure in MySQL stored procedures, namely while and repeat-until statements.

1. while loop statement

while loop statement is a basic loop structure. In the MySQL stored procedure, you can use the while statement to implement repeated execution of a piece of code. until the termination condition is met. The syntax of while is as follows:

WHILE condition DO
    statements
END WHILE;

Among them, condition is the condition to terminate the loop, and statements are the code blocks that need to be executed. When condition is true, the block of code is executed repeatedly until condition becomes false. The following is a simple while loop example:

DELIMITER $$
CREATE PROCEDURE test_while()
BEGIN
    DECLARE i INT DEFAULT 1;
    WHILE i <= 10 DO
        SELECT CONCAT('Hello, World!', i);
        SET i = i + 1;
    END WHILE;
END$$
DELIMITER ;

In the above example, we define a stored procedure test_while, which declares an integer variable i with a value of 1. Then use a while loop to execute the query statement and output the results. After each loop ends, the value of i will be increased by 1 until i is greater than 10 and the loop will stop. Executing the stored procedure test_while, the results are as follows:

+----------------------+
| CONCAT('Hello, World!', i) |
+----------------------+
| Hello, World!1       |
| Hello, World!2       |
| Hello, World!3       |
| Hello, World!4       |
| Hello, World!5       |
| Hello, World!6       |
| Hello, World!7       |
| Hello, World!8       |
| Hello, World!9       |
| Hello, World!10      |
+----------------------+

We can see that the loop is executed 10 times, and a string with the value of i is output every time it is executed.

2. repeat-until loop statement

repeat-until loop statement is another commonly used loop structure. In MySQL stored procedures, it can be implemented using the repeat-until statement. The repeated execution of a block of code until a termination condition is met. The syntax of repeat-until is as follows:

REPEAT
    statements
UNTIL condition
END REPEAT;

Among them, statements are the code blocks that need to be executed, and condition is the condition to terminate the loop. The loop terminates when condition is true, and statements are executed repeatedly if condition is false. The following is a simple repeat-until loop example:

DELIMITER $$
CREATE PROCEDURE test_repeat()
BEGIN
    DECLARE i INT DEFAULT 1;
    REPEAT
        SELECT CONCAT('Hello, World!', i);
        SET i = i + 1;
    UNTIL i > 10
    END REPEAT;
END$$
DELIMITER ;

In the above example, we define a stored procedure test_repeat, which declares an integer variable i with a value of 1. Then use a repeat-until loop to execute the query statement and output the results. After each loop ends, the value of i will be increased by 1 until i is greater than 10 and the loop will stop. Execute the stored procedure test_repeat, and the results are as follows:

+----------------------+
| CONCAT('Hello, World!', i) |
+----------------------+
| Hello, World!1       |
| Hello, World!2       |
| Hello, World!3       |
| Hello, World!4       |
| Hello, World!5       |
| Hello, World!6       |
| Hello, World!7       |
| Hello, World!8       |
| Hello, World!9       |
| Hello, World!10      |
+----------------------+

We can see that the loop is executed 10 times, and a string with the i value is output every time it is executed.

3. Summary

Loop is an indispensable basic structure in programming. The while and repeat-until statements in MySQL stored procedures can help us realize loop control, thus Complete data processing and analysis tasks more efficiently. In practice, you need to pay attention to the setting and updating of loop conditions, as well as the execution order and efficiency of statements inside the loop, to ensure the correctness and performance of the code.

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