MySQL流程控制语句简介

备注:测试数据库版本为MySQL 8.0

这个blog我们来聊聊MySQL流程控制语句，流程控制语句在存储过程、函数、触发器和事件里面可以被使用到。

一.CASE语句

CASE语句用于条件判断
CASE语句与CASE函数使用上有差异:
1.CASE函数是在SQL里使用，而CASE语句在存储过程及函数里使用
2.CASE语句不能用esle null子句,用else begin end; 空的程序块代替
3.CASE函数的结尾是END，而CASE语句结尾是END CASE

语法:

-- 语法1:
CASE case_value
    WHEN when_value THEN statement_list
    [WHEN when_value THEN statement_list] ...
    [ELSE statement_list]
END CASE
-- 语法2:
CASE
    WHEN search_condition THEN statement_list
    [WHEN search_condition THEN statement_list] ...
    [ELSE statement_list]
END CASE

案例:

DELIMITER |

CREATE PROCEDURE p(in pi_num int)
  BEGIN

    CASE pi_num
      WHEN 2 THEN SELECT 'TWO';
      WHEN 3 THEN SELECT 'THREE';
      ELSE
        BEGIN
        END;
    END CASE;
  END;
|

DELIMITER ;

mysql> DELIMITER |
mysql>
mysql> CREATE PROCEDURE p(in pi_num int)
    ->   BEGIN
    ->
    ->     CASE pi_num
    ->       WHEN 2 THEN SELECT 'TWO';
    ->       WHEN 3 THEN SELECT 'THREE';
    ->       ELSE
    ->         BEGIN
    ->         END;
    ->     END CASE;
    ->   END;
    -> |
Query OK, 0 rows affected (0.04 sec)

mysql>
mysql> DELIMITER ;
mysql>
mysql> CALL P(2);
+-----+
| TWO |
+-----+
| TWO |
+-----+
1 row in set (0.03 sec)

Query OK, 0 rows affected (0.04 sec)

mysql> CALL P(3);
+-------+
| THREE |
+-------+
| THREE |
+-------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.01 sec)

mysql> CALL P(4);
Query OK, 0 rows affected (0.00 sec)

二.IF语句

IF语句用于条件判断
IF语句和IF函数差别比较大，可以参考我之前的blog

语法:

IF search_condition THEN statement_list
    [ELSEIF search_condition THEN statement_list] ...
    [ELSE statement_list]
END IF

案例:

DELIMITER //

CREATE PROCEDURE P2(in pi_a INT,in pi_b INT)

  BEGIN
    DECLARE l_result VARCHAR(20);

    IF pi_a > pi_b THEN SET l_result = '>';
    ELSEIF pi_a = pi_b THEN SET l_result = '=';
    ELSE SET l_result = '<';
    END IF;

    SET l_result = CONCAT(pi_a, ' ', l_result, ' ', pi_b);

    select l_result as result;
  END //

DELIMITER ;

mysql> DELIMITER //
mysql>
mysql> CREATE PROCEDURE P2(in pi_a INT,in pi_b INT)
    ->
    ->   BEGIN
    ->     DECLARE l_result VARCHAR(20);
    ->
    ->     IF pi_a > pi_b THEN SET l_result = '>';
    ->     ELSEIF pi_a = pi_b THEN SET l_result = '=';
    ->     ELSE SET l_result = '<';
    ->     END IF;
    ->
    ->     SET l_result = CONCAT(pi_a, ' ', l_result, ' ', pi_b);
    ->
    ->     select l_result as result;
    ->   END //
Query OK, 0 rows affected (0.01 sec)

mysql>
mysql> DELIMITER ;
mysql>
mysql> call p2(1,2);
+--------+
| result |
+--------+
| 1 < 2  |
+--------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.01 sec)

mysql> call p2(2,2);
+--------+
| result |
+--------+
| 2 = 2  |
+--------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.01 sec)

mysql> call p2(3,2);
+--------+
| result |
+--------+
| 3 > 2  |
+--------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.01 sec)

三.ITERATE语句

ITERATE迭代语句
语法:

ITERATE label

ITERATE只能和LOOP，REPEAT，WHILE语句配合使用，作用是再次启动循环。

四.LEAVE语句

LEAVE退出语句
语法:

LEAVE label

LEAVE语句退出循环或程序块，只能和begin … end，LOOP，REPEAT，WHILE语句配合使用

五.LOOP语句

LOOP循环语句
语法:

[begin_label:] LOOP
    statement_list
END LOOP [end_label]

案例:用LOOP循环求1到10之和

DELIMITER //

CREATE PROCEDURE p3()
BEGIN
 
  declare sum int default 0;
  declare l_num int default 1;

  -- loop_1为接下来loop循环的标签
  loop_1: LOOP
    SET sum = sum + l_num;
    SET l_num = l_num + 1;
    IF l_num <= 10 THEN
      -- 条件成立,继续循环 
      ITERATE loop_1;
    END IF;
    -- 退出循环
    LEAVE loop_1;
  END LOOP loop_1;
  
  select  sum;

END;
//

DELIMITER ;

mysql> DELIMITER //
mysql>
mysql> CREATE PROCEDURE p3()
    -> BEGIN
    ->
    ->   declare sum int default 0;
    ->   declare l_num int default 1;
    ->
    ->   -- loop_1为接下来loop循环的标签
    ->   loop_1: LOOP
    ->     SET sum = sum + l_num;
    ->     SET l_num = l_num + 1;
    ->     IF l_num <= 10 THEN
    ->       -- 条件成立,继续循环
    ->       ITERATE loop_1;
    ->     END IF;
    ->     -- 退出循环
    ->     LEAVE loop_1;
    ->   END LOOP loop_1;
    ->
    ->   select  sum;
    ->
    -> END;
    -> //
Query OK, 0 rows affected (0.01 sec)

mysql>
mysql> DELIMITER ;
mysql>
mysql> call p3;
+------+
| sum  |
+------+
|   55 |
+------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.01 sec)

六.REPEAT语句

REPEAT语句一直重复，直到条件不满足
语法:

[begin_label:] REPEAT
    statement_list
UNTIL search_condition
END REPEAT [end_label]

案例:

DELIMITER //

CREATE PROCEDURE p4()
BEGIN
  
  declare l_n1 int default 10;
  declare l_n2 int default 1;
  
  -- 重复给l_n2加1,直到l_n2大于l_n1
  repeat 
     set l_n2 = l_n2 + 1;
  until l_n2 > l_n1 end repeat;
  
  select l_n2;  

END;
//

DELIMITER ;

mysql> DELIMITER //
mysql>
mysql> CREATE PROCEDURE p4()
    -> BEGIN
    ->
    ->   declare l_n1 int default 10;
    ->   declare l_n2 int default 1;
    ->
    ->   -- 重复给l_n2加1,直到l_n2大于l_n1
    ->   repeat
    ->      set l_n2 = l_n2 + 1;
    ->   until l_n2 > l_n1 end repeat;
    ->
    ->   select l_n2;
    ->
    -> END;
    -> //
Query OK, 0 rows affected (0.01 sec)

mysql>
mysql> DELIMITER ;
mysql>
mysql>
mysql> call p4;
+------+
| l_n2 |
+------+
|   11 |
+------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.01 sec)

七.WHILE语句

语法:

[begin_label:] WHILE search_condition DO
    statement_list
END WHILE [end_label]

案例:

DELIMITER //

CREATE PROCEDURE p5()
BEGIN
  
  declare l_n1 int default 10;
 
  
  while l_n1 > 0 DO  
     set l_n1 = l_n1 - 1;
  end while;
  
  select l_n1;  

END;
//

DELIMITER ;

mysql> DELIMITER //
mysql>
mysql> CREATE PROCEDURE p5()
    -> BEGIN
    ->
    ->   declare l_n1 int default 10;
    ->
    ->
    ->   while l_n1 > 0 DO
    ->      set l_n1 = l_n1 - 1;
    ->   end while;
    ->
    ->   select l_n1;
    ->
    -> END;
    -> //
Query OK, 0 rows affected (0.04 sec)

mysql>
mysql> DELIMITER ;
mysql>
mysql> call p5;
+------+
| l_n1 |
+------+
|    0 |
+------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.01 sec)