块定义
| [label:] BEGIN variable and condition declarations cursor declarations handler declarations program code END[label]; |
可使用 LEAVE [label]来跳出块
例:
mysql> CREATE PROCEDURE nested_blocks5( )
outer_block: BEGIN
DECLARE l_status int;
SET l_status=1;
inner_block: BEGIN
IF (l_status=1) THEN
LEAVE inner_block;
END IF;
SELECT 'This statement will never be executed';
END inner_block;
SELECT 'End of program';
END outer_block
结果:
mysql> CALL nested_blocks5( )$$
+----------------+
| End of program |
+----------------+
| End of program |
注：如果在命令行建立存储过程，可能需要用DELIMITER 命令将改变默认的命令结束符;改为其它符号，如 DELIMITER $$
块的嵌套:块可以嵌套出现，但需注意其中变量的有效范围
1、 块内声明的变量，在块的外部是不存在的
mysql> CREATE PROCEDURE nested_blocks1( )
BEGIN
BEGIN
DECLARE inner_variable VARCHAR(20);
SET inner_variable='This is my private data';
END;
SELECT inner_variable;
END;
结果:
mysql> CALL nested_blocks1( )
ERROR 1054 (42S22): Unknown column 'inner_variable' in 'field list'
2、 块内可以覆盖块外声明的同名变量
mysql> CREATE PROCEDURE nested_blocks2( )
BEGIN
DECLARE my_variable varchar(20);
SET my_variable='This value was set in the outer block';
BEGIN
SET my_variable='This value was set in the inner block';
END;
SELECT my_variable, 'Changes in the inner block are visible in the outer block';
END;
结果:
mysql> CALL nested_blocks2( )
+---------------------+-----------------------------------------------------------+
| my_variable | Changes in the inner block are visible in the outer block |
+---------------------+-----------------------------------------------------------+
| This value was set | |
| in the inner block | Changes in the inner block are visible in the outer block |
+---------------------+-----------------------------------------------------------+
3、 块内声明的变量的改变，不会影响到块外声明的同名变量
mysql> CREATE PROCEDURE nested_blocks3( )
BEGIN
DECLARE my_variable varchar(20);
SET my_variable='This value was set in the outer block';
BEGIN
DECLARE my_variable VARCHAR(20);
SET my_variable='This value was set in the inner block';
END;
SELECT my_variable, 'Can''t see changes made in the inner block';
END;
结果:
mysql> CALL nested_blocks3( )$$
+---------------------------+-------------------------------------------+
| my_variable | Can't see changes made in the inner block |
+---------------------------+-------------------------------------------+
| This value was set in the | |
| outer block | Can't see changes made in the inner block |
+---------------------------+-------------------------------------------+
条件语句
IF THEN语句
| IF expression THEN commands [ELSEIF expression THEN commands ....] [ELSE commands] END IF; |
该语句可以嵌套
例:
IF (sale_value > 200) THEN CALL free_shipping(sale_id);
IF (customer_status='PLATINUM') THEN
CALL apply_discount(sale_id,20);
ELSEIF (customer_status='GOLD') THEN
CALL apply_discount(sale_id,15);
ELSE CALL apply_discount(sale_id,5);
END IF;
END IF;
Case语句
Case语句有两种语法
语法一:
| CASE expression WHEN value THEN statements [WHEN value THEN statements ...] [ELSE statements] END CASE; |
语法二:
| CASE WHEN condition THEN statements [WHEN condition THEN statements...] [ELSE statements] END CASE; |
循环语句
loop循环
| [label:] LOOP statements END LOOP [label]; |
loop循环不会自动结束，须用LEAVE来跳出, 同样地，使用ITERATE label;来执行下一循环，类似一般编程语言的continue
例
SET i=0;
loop1: LOOP
SET i=i+1;
IF i>=10 THEN LEAVE loop1;
ELSEIF MOD(i,2)=0 THEN ITERATE loop1;
END IF;
SELECT CONCAT(i," is an odd number");
END LOOP loop1;
REPEAT ... UNTIL循环
| [label:] REPEAT statements UNTIL expression END REPEAT [label] |
当expression为true时跳出，效果同:
| some_label:LOOP statements IF expression THEN LEAVE some_label; END IF; END LOOP; |
While循环
| [label:] WHILE expression DO statements END WHILE [label] |
类似REPEAT .. UNTIL,区别在于while先判断条件再执行语句，REPEAT..UNTIL先执行语句，再判断条件