在存儲過程中使用變量

Question

試圖找出它爲什麼是NULL。我期待7印。

mysql> set @total = 0; 
Query OK, 0 rows affected (0.00 sec) 

mysql> call getAuthorCount(@total); 
+------------------------+ 
| count(distinct author) | 
+------------------------+ 
|      7 | 
+------------------------+ 
1 row in set (0.00 sec) 

Query OK, 0 rows affected (0.02 sec) 

mysql> select @total as totalauthors; 
+--------------+ 
| totalauthors | 
+--------------+ 
|   NULL | 
+--------------+ 

的過程，

mysql> create procedure getAuthorCount(out authorcount int) 
    -> begin 
    -> select count(distinct author) from libbooks; 
    -> end 
    -> // 

Answer 1

應該使用INOUT參數 -

CREATE PROCEDURE getAuthorCount(INOUT authorcount INT) 
BEGIN 
    SELECT count(DISTINCT author) FROM libbooks; 
END 

---

實例：

當@total值作爲是（0在，0 out）：

DROP PROCEDURE getAuthorCount; 
DELIMITER $$ 
CREATE PROCEDURE getAuthorCount(INOUT authorcount INT) 
BEGIN 
    -- SET authorcount = 100; 
END$$ 
DELIMITER ; 

SET @total = 0; 
CALL getAuthorCount(@total); 
SELECT @total AS totalauthors; 
+--------------+ 
| totalauthors | 
+--------------+ 
|   0 | 
+--------------+ 

當@total值替換存儲過程中新的價值：

DROP PROCEDURE getAuthorCount; 
DELIMITER $$ 
CREATE PROCEDURE getAuthorCount(OUT authorcount INT) 
BEGIN 
    SET authorcount = 100; 
END$$ 
DELIMITER ; 

SET @total = 0; 
CALL getAuthorCount(@total); 
SELECT @total AS totalauthors; 
+--------------+ 
| totalauthors | 
+--------------+ 
|   100 | 
+--------------+