Question

我有一个包含值的表（字段值）：

CREATE TABLE values (
    questionario_id INT(10) UNSIGNED NOT NULL,
    insegnamento_id INT(11) NOT NULL,
    domanda_id TINYINT(3) UNSIGNED NOT NULL,
    valore TINYINT(3) UNSIGNED NOT NULL COMMENT '1,2,3,4',
...

我想计算具有值（valore）1的所有行以及具有值2,3和4的所有行.WHERE子句是常见的。

我可以UNION：

SELECT
    COUNT(V.valore) AS valori_1
FROM
        values V
JOIN    questionari Q ON (V.questionario_id = Q.id) 
JOIN    sessioni S ON (Q.sessione_id = S.id)
WHERE   V.domanda_id = 1 AND S.anno_accademico = 0 AND S.settimana = 0
AND V.valore=1
UNION SELECT
    COUNT(V.valore) AS valori_2
FROM
        values V
JOIN    questionari Q ON (V.questionario_id = Q.id) 
JOIN    sessioni S ON (Q.sessione_id = S.id)
WHERE   V.domanda_id = 1 AND S.anno_accademico = 0 AND S.settimana = 0
AND V.valore=2
UNION
...

我不喜欢这个，因为我必须重复整个连接和WHERE子句。

出于同样的原因，我不喜欢这种形式：

SELECT
    (SELECT COUNT(V.valore) FROM values V WHERE ...) AS C1,
    (SELECT COUNT(V.valore) FROM values V WHERE ...) AS C2,
    (SELECT COUNT(V.valore) FROM values V WHERE ...) AS C3,
    ...

尝试了CASE选项：

 SELECT
        COUNT(CASE WHEN V.valore=1 THEN 1 ELSE 0 END) AS C1,
        COUNT(CASE WHEN V.valore=2 THEN 1 ELSE 0 END) AS C2,
        COUNT(CASE WHEN V.valore=3 THEN 1 ELSE 0 END) AS C3
    FROM
            values V
    JOIN    questionari Q ON (V.questionario_id = Q.id) 
    JOIN    sessioni S ON (Q.sessione_id = S.id)
    WHERE
        V.domanda_id = 1
    AND 
        S.anno_accademico = 0
    AND 
        S.settimana = 0

但它不起作用。任何提示？

Answer 1

group by不会帮助你吗？

SELECT V.valore, COUNT(V.valore) AS valori_1
FROM
        values V
JOIN    questionari Q ON (V.questionario_id = Q.id) 
JOIN    sessioni S ON (Q.sessione_id = S.id)
WHERE   V.domanda_id = 1 AND S.anno_accademico = 0 AND S.settimana = 0
GROUP BY V.valore

Answer 2

对于透视版本

使用

SUM(CASE WHEN V.valore=1 THEN 1 ELSE 0 END)

或者

COUNT(CASE WHEN V.valore=1 THEN 1  END) /*ELSE NULL is implicit*/

不

COUNT(CASE WHEN V.valore=1 THEN 1 ELSE 0 END)

COUNT计算所有非空值，1和0均为NOT NULL，因此您的代码实际上只是重复COUNT(*)三次。

SQL计数具有不同值的行

2 个答案: