MySQL存储过程之代码块、条件控制、迭代

ouzhoudijie

1，代码块
代码顺序

1，变量和条件声明
2，Cursor声明
3，Handler声明
4，程序代码

可以给代码块加lebel，这样END匹配比较直观，还可以用LEAVE语句来终结代码块:

[label:] BEGIN
varaiable and condition declarations
cursor declarations
handler declarations
program code
END [label];

代码块可以嵌套:

CREATE PROCEDURE nested_blocks()
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;

LEAVE的例子:

CREATE PROCEDURE nested_blocks()
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;


2，条件控制
IF:

IF expression THEN commands
[ELSEIF expression THEN commands]
[ELSE commands]
END IF;

例子:

IF (sale_value > 200) THEN
CALL free_shipping(sale_id);    /*Free shipping*/
IF (customer_status='PLATINUM') THEN
CALL apply_discount(sale_id,20); /* 20% discount */
ELSEIF (customer_status='GOLD') THEN
CALL apply_discount(sale_id,15); /* 15% discount */
ELSEIF (customer_status='SILVER') THEN
CALL apply_discount(sale_id,10); /* 10% discount */
ELSEIF (customer_status='BRONZE') THEN
CALL apply_discount(sale_id,5); /* 5% discount*/
END IF;
END IF;

CASE:

CASE
WHEN condition THEN
statements
[WHEN condition THEN
statements...]
[ELSE
statements]
END CASE;

例子:

CASE
WHEN (sale_value>200) THEN
CALL free_shipping(sale_id);
CASE customer_status
WHEN 'PLATINUM' THEN
CALL apply_discount(sale_id,20);
WHEN 'GOLD' THEN
CALL apply_discount(sale_id,15);
WHEN 'SILVER' THEN
CALL apply_discount(sale_id,10);
WHEN 'BRONZE' THEN
CALL apply_discount(sale_id,5);
END CASE;
END CASE;

CASE与SELECT语句结合的妙用:

SELECT (CASE WHEN (t.a = 1 AND t.b = 0) THEN t.c ELSE 'N/A' END) AS result FROM test t order by result asc


3，迭代
LOOP

[label:] LOOP
statements
END LOOP [label];

REPEAT...UNTIL

[label:] REPEAT
statements
UNTIL expression
END REPEAT [label]

WHILE

[label:] WHILE expression DO
statements
END WHILE [label]

LEAVE语句

SET i=1;
myloop: LOOP
SET i=i+1;
IF i=10 then
LEAVE myloop;
END IF:
END LOOP myloop;
SELECT 'I can count to 10';

ITERATE语句

SET i=0;
loop1: LOOP
SET i=i+1;
IF i>=10 THEN                 /*Last number - exit loop*/
LEAVE loop1;
ELSEIF MOD(i, 2)=0 THEN       /*Even number - try again*/
ITERATE loop1;
END IF;
SELECT CONCAT(i, " is an odd number");
END LOOP loop1;

嵌套循环

DECLARE i, j INT DEFAULT 1;
outer_loop: LOOP
SET j=1;
inner_loop: LOOP
SELECT concat(i, " times ", j, " is ", i*j);
SET j=j+1;
IF j>12 THEN
LEAVE inner_loop;
END IF;
END LOOP inner_loop;
SET i=i+1;
IF i>12 THEN
LEAVE outer_loop;
END IF;
END LOOP outer_loop;