MySQL Case Statement - 其中的未知列

Question

我的查询有问题，出了什么问题？

SELECT 
    CASE 
        WHEN cp_pessoa.score < 100 THEN 1 
        WHEN cp_pessoa.score < 300 THEN 2 
        WHEN cp_pessoa.score >= 300 THEN 3 
    END as id_ranking
FROM cp_pessoa 
    WHERE id_ranking IN (1,2);

我收到此错误：＃1054 - 'where子句'中的未知列'id_ranking'

谢谢！

Answer 1

这是因为SELECT准备结果（作为id_ranking）比调用WHERE更早。

SELECT 
    CASE 
    WHEN cp_pessoa.score < 100 THEN 1 
    WHEN cp_pessoa.score < 300 THEN 2 
    WHEN cp_pessoa.score >= 300 THEN 3 
END as id_ranking
FROM cp_pessoa 
WHERE cp_pessoa.score < 300;

这是如何工作的（订单）：

1. FROM clause
2. WHERE子句
3. GROUP BY子句
4. HAVING条款
5. SELECT条款
6. ORDER BY子句

Answer 2

where中不允许使用别名。将查询更改为

SELECT * FROM 
(
SELECT 
    CASE 
        WHEN cp_pessoa.score < 100 THEN 1 
        WHEN cp_pessoa.score < 300 THEN 2 
        WHEN cp_pessoa.score >= 300 THEN 3 
    END as id_ranking
FROM cp_pessoa 
)a 
WHERE id_ranking IN (1,2)

或

SELECT 
    CASE 
        WHEN cp_pessoa.score < 100 THEN 1 
        WHEN cp_pessoa.score < 300 THEN 2 
        WHEN cp_pessoa.score >= 300 THEN 3 
    END as id_ranking
FROM cp_pessoa 
WHERE 
  CASE 
        WHEN cp_pessoa.score < 100 THEN 1 
        WHEN cp_pessoa.score < 300 THEN 2 
        WHEN cp_pessoa.score >= 300 THEN 3 
    END IN (1,2)

Answer 3

您不能在where子句中使用别名，而是尝试

SELECT 
CASE 
    WHEN cp_pessoa.score < 100 THEN 1 
    WHEN cp_pessoa.score < 300 THEN 2 
    WHEN cp_pessoa.score >= 300 THEN 3 
END as id_ranking
FROM cp_pessoa 
WHERE cp_pessoa.score < 300