mysql - 如何在连接时删除 NULL 结果

Question

我有以下查询：

SELECT concat(first_name, ' ', last_name) as full_name FROM auth_user

我将如何做相当于：

SELECT concat(first_name, ' ', last_name) as full_name FROM auth_user 
    **WHERE full_name IS NOT NULL**

（产生一个Unknown column 'full_name' in 'where clause'）

score 2 · Accepted Answer

解决这个问题的两种方法：

SELECT concat(first_name, ' ', last_name) as full_name FROM auth_user 
WHERE first_name IS NOT NULL and last_name is not null

或者，

select * from (
    SELECT concat(first_name, ' ', last_name) as full_name FROM auth_user)
where full_name is not null

score 0 · Accepted Answer

SELECT concat(IF(first_name IS NOT NULL, first_name, ''), ' ', IF(last_name IS NOT NULL, last_name, '')) as full_name FROM auth_user;

在 where 子句中使用 IS NOT NULL 和在 IF 子句中使用 IS NOT NULL 的区别在于 where 子句将消除（过滤）具有空值的行，而 if 子句将获取所有行但具有 full_name不是“未知”，它只会丢失空组件

2 回答 2