如何從mysql表中選擇隨機列？

Question

我的桌子是這樣的-

+---------+---------------+------+-----+---------+-------+
| PK      | ToicID        |topic1|topic2| topic3 |topic4 |
+---------+---------------+------+-----+---------+-------+
| 1       | 1             | abc  | xyz  | mno    | pqr   |
+---------+---------------+------+-----+---------+-------+

我用這個查詢

select * from table order by rand() limit 1

它只給我隨機行。 但我想隨機選擇任何主題，如下所示

Topic3
|mno| 

==========================================

但條件是該列不應為空或為null。

Answer 1

請問您是否可以查詢一下？

SELECT  CASE FLOOR(RAND()*4+1)
             WHEN 1 THEN TOPIC1 
             WHEN 2 THEN TOPIC2 
             WHEN 3 THEN TOPIC3 
             WHEN 4 THEN TOPIC4 END AS TOPIC_RAND
FROM TABLE;

注釋后更新了版本，並添加了相關的子句-NULL / blank：

   select version() as 'mysql version';

SELECT  CASE FLOOR(RAND()*4+1)   /*wrote here just 1 to make tests */
    WHEN 1 THEN COALESCE(TOPIC1, TOPIC2, TOPIC3, TOPIC4)  
    WHEN 2 THEN COALESCE(TOPIC2, TOPIC3, TOPIC4, TOPIC1) 
    WHEN 3 THEN COALESCE(TOPIC3, TOPIC4, TOPIC1, TOPIC2) 
    WHEN 4 THEN COALESCE(TOPIC4, TOPIC1, TOPIC2, TOPIC3) END AS TOPIC_RAND
FROM (SELECT NULLIF(TOPIC1,'') AS TOPIC1, NULLIF(TOPIC2,'') AS TOPIC2,NULLIF(TOPIC3,'') AS TOPIC3,NULLIF(TOPIC4,'') AS TOPIC4          
      FROM TTE1) A;

樣本數據

CREATE TABLE TTE1 (TOPIC1 VARCHAR(10), TOPIC2 VARCHAR(10), TOPIC3 VARCHAR(10), TOPIC4 VARCHAR(10));
INSERT INTO TTE1 VALUES ('abc','cde','efg','hij');
INSERT INTO TTE1 VALUES (NULL,'k2','k3','k4');
INSERT INTO TTE1 VALUES ('','i2','i3','i4');

輸出：

mysql version
1   5.7.12-log

TOPIC_RAND
1   cde
2   k4
3   i2

Answer 2

規范化的架構可能如下所示（代碼段）：

topic_id topic_name 
       1 abc  
       2 xyz  
       3 mno  
       4 pqr  

從這里開始，問題變得微不足道了。