学步园

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;