MySQL流程控制语句解读

IF condition THEN
	...
ELSE condition THEN
	...
ELSE
	...
END IF

-- 创建存储过程
CREATE PROCEDURE example_if (IN x INT)
BEGIN
	IF x = 1 THEN
		SELECT 1;
	ELSEIF x = 2 THEN 
		SELECT 2;
	ELSE
		SELECT 3;
	END IF;
END;

-- 调用存储过程
CALL example_if(2);

CASE value
	WHEN value THEN ...
	WHEN value THEN ...
	ELSE ...
END CASE

CASE
	WHEN value THEN ...
	WHEN value THEN ...
	ELSE ...
END CASE

-- 创建存储过程
CREATE PROCEDURE example_case(IN x INT)
BEGIN
	CASE x
		WHEN 1 THEN SELECT 1;
		WHEN 2 THEN SELECT 2;
		ELSE SELECT 3;
	END CASE;
END;

-- 调用存储过程
CALL example_case(5);

WHILE condition DO
...
END WHILE;

-- 创建存储过程
CREATE PROCEDURE example_while(OUT sum INT)
BEGIN
	DECLARE i INT DEFAULT 1;
	DECLARE s INT DEFAULT 0;

	WHILE i <= 100 DO
		SET s = s+i;
		SET i = i+1;
	END WHILE;

	SET sum = s;
END;

-- 调用存储过程
CALL example_while(@sum);
SELECT @sum;

LOOP
	...
END LOOP

LEAVE label

-- 创建存储过程
CREATE PROCEDURE example_loop(OUT sum INT)
BEGIN
	DECLARE i INT DEFAULT 1;
	DECLARE s INT DEFAULT 0;
	
	loop_label:LOOP
		SET s = s+i;
		SET i = i+1;
	
		IF i>100 THEN
			-- 退出LOOP循环
			LEAVE loop_label;  
		END IF;
	END LOOP;

	SET sum = s;
END;

-- 调用存储过程
CALL example_loop(@sum);
SELECT @sum;

REPEAT
	...
	UNTIL condition
END REPEAT

-- 创建存储过程
CREATE PROCEDURE example_repeat(OUT sum INT)
BEGIN
	DECLARE i INT DEFAULT 1;
	DECLARE s INT DEFAULT 0;

	REPEAT
		SET s = s+i;
		SET i = i+1;
		
		UNTIL i > 100
	END REPEAT;
	
	SET sum = s;
END;

-- 调用存储过程
CALL example_repeat(@sum);
SELECT @sum;

ITERATE label

-- 创建存储过程
CREATE PROCEDURE example_iterate(OUT sum INT)
BEGIN
	DECLARE i INT DEFAULT 0;
	DECLARE s INT DEFAULT 0;
	
	loop_label:LOOP

		SET i = i+1;		

		IF i > 10 THEN
			-- 退出整个循环
			LEAVE loop_label;  
		END IF;

		IF (i mod 2) THEN
			SET s = s+i;
		ELSE
			-- 退出本次循环，继续下一个循环
			ITERATE  loop_label;
		END IF;

	END LOOP;

	SET sum = s;
END;

-- 调用存储过程
CALL example_iterate(@sum);
SELECT @sum;