MySQL SELECT仅非空值 [英] MySQL SELECT only not null values

问题描述

是否可以执行仅包含NOT NULL值的select语句?

Is it possible to do a select statement that takes only NOT NULL values?

现在我正在使用它:

SELECT * FROM table

然后我必须用php循环过滤掉空值.

And then I have to filter out the null values with a php loop.

有没有办法做

SELECT * (that are NOT NULL) FROM table

?

现在，当我选择*时，我得到val1，val2，val3，null，val4，val5，null，null等.但是我只想获取结果中不为null的值.不用循环过滤就可以吗?

Right now when I select * I get val1,val2,val3,null,val4,val5,null,null etc.... but I just want to get the values that are not null in my result. Is this possible without filtering with a loop?

推荐答案

您应使用IS NOT NULL. (比较运算符=和<>都在表达式的任一侧给出UNKNOWN和NULL.)

You should use IS NOT NULL. (The comparison operators = and <> both give UNKNOWN with NULL on either side of the expression.)

SELECT * 
FROM table 
WHERE YourColumn IS NOT NULL;

出于完整性考虑，我将提到在MySQL中，您还可以否定

Just for completeness I'll mention that in MySQL you can also negate the null safe equality operator but this is not standard SQL.

SELECT *
FROM table 
WHERE NOT (YourColumn <=> NULL);

经过编辑以反映评论.听起来您的表格可能未采用第一范式，在这种情况下，更改结构可能会使您的工作更轻松.不过，还有其他几种方法...

Edited to reflect comments. It sounds like your table may not be in first normal form in which case changing the structure may make your task easier. A couple of other ways of doing it though...

SELECT val1 AS val
FROM  your_table
WHERE val1 IS NOT NULL
UNION ALL
SELECT val2 
FROM  your_table
WHERE val2 IS NOT NULL
/*And so on for all your columns*/

上面的缺点是，它为每列多次扫描表一次.下面可能会避免这种情况，但是我还没有在MySQL中进行测试.

The disadvantage of the above is that it scans the table multiple times once for each column. That may possibly be avoided by the below but I haven't tested this in MySQL.

SELECT CASE idx
         WHEN 1 THEN val1
         WHEN 2 THEN val2
       END AS val
FROM   your_table
        /*CROSS JOIN*/
       JOIN (SELECT 1 AS idx
                   UNION ALL
                   SELECT 2) t
HAVING val IS NOT NULL  /*Can reference alias in Having in MySQL*/

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