Question

我有以下查询，它是由PHP函数生成的，它变得非常复杂但到目前为止工作正常。

如果第三个字段等于1，我想要做的是通过第二个字段覆盖一个字段。

所以当 table4 .cpda_meta_val等于1时，我希望 table2 .cpda_meta_val实际上等于 table3 .cpda_meta_val，这样我就可以了在WHERE / ORDER BY子句中使用结果值table2.cpda_meta_val。

我希望到目前为止我已经有道理了。

以下是原始查询：

SELECT
    mainTable.cpda_id 
FROM
    cp_data AS mainTable 
LEFT OUTER JOIN
    cp_data AS table0 
        ON table0.cpda_meta_id = mainTable.cpda_id 
        AND table0.cpda_meta_key = '47248' 
LEFT OUTER JOIN
    cp_data AS table1 
        ON table1.cpda_meta_id = mainTable.cpda_id 
        AND table1.cpda_meta_key = '47241' 
LEFT OUTER JOIN
    cp_data AS table2 
        ON table2.cpda_meta_id = mainTable.cpda_id 
        AND table2.cpda_meta_key = '47242' 
LEFT OUTER JOIN
    cp_data AS table3 
        ON table3.cpda_meta_id = mainTable.cpda_id 
        AND table3.cpda_meta_key = '66838' 
LEFT OUTER JOIN
    cp_data AS table4 
        ON table4.cpda_meta_id = mainTable.cpda_id 
        AND table4.cpda_meta_key = '66843' 
LEFT OUTER JOIN
    cp_data AS table5 
        ON table5.cpda_meta_id = mainTable.cpda_id 
        AND table5.cpda_meta_key = '47252' 
LEFT OUTER JOIN
    cp_data AS table6 
        ON table6.cpda_meta_id = mainTable.cpda_id 
        AND table6.cpda_meta_key = '47255' 
WHERE
    mainTable.cpda_meta_id = 47236 
    AND mainTable.cpda_active = 1 
    AND mainTable.cpda_meta_key = 'set_element_stub' 
    AND table0.cpda_meta_val = 49297 
    AND table1.cpda_meta_val != 66116 
    AND (
        table5.cpda_meta_val = 497 
        OR table6.cpda_meta_val = 497
    ) 
ORDER BY
    table2.cpda_meta_val,
    mainTable.cpda_seq,
    mainTable.cpda_id

这是我尝试实现上述目标，但不起作用并且不检索任何行：

SELECT
    mainTable.cpda_id 
FROM
    cp_data AS mainTable 
LEFT OUTER JOIN
    cp_data AS table0 
        ON table0.cpda_meta_id = mainTable.cpda_id 
        AND table0.cpda_meta_key = '47248' 
LEFT OUTER JOIN
    cp_data AS table1 
        ON table1.cpda_meta_id = mainTable.cpda_id 
        AND table1.cpda_meta_key = '47241' 
LEFT OUTER JOIN
    cp_data AS table2 
        ON table2.cpda_meta_id = mainTable.cpda_id 
        AND table2.cpda_meta_key = '47242' 
LEFT OUTER JOIN
    cp_data AS table3 
        ON table3.cpda_meta_id = mainTable.cpda_id 
        AND table3.cpda_meta_key = '66838' 
LEFT OUTER JOIN
    cp_data AS table4 
        ON table4.cpda_meta_id = mainTable.cpda_id 
        AND table4.cpda_meta_key = '66843' 
LEFT OUTER JOIN
    cp_data AS table5 
        ON table5.cpda_meta_id = mainTable.cpda_id 
        AND table5.cpda_meta_key = '47252' 
LEFT OUTER JOIN
    cp_data AS table6 
        ON table6.cpda_meta_id = mainTable.cpda_id 
        AND table6.cpda_meta_key = '47255'
CASE 
    WHEN table4.cpda_meta_val = 1 THEN table3.cpda_meta_val 
    ELSE table2.cpda_meta_val 
END as table2.cpda_meta_val 
WHERE
    mainTable.cpda_meta_id = 47236 
    AND mainTable.cpda_active = 1 
    AND mainTable.cpda_meta_key = 'set_element_stub' 
    AND table0.cpda_meta_val = 49297 
    AND table1.cpda_meta_val != 66116 
    AND (
        table5.cpda_meta_val = 497 
        OR table6.cpda_meta_val = 497
    ) 
ORDER BY
    table2.cpda_meta_val,
    mainTable.cpda_seq,
    mainTable.cpda_id

如果有人能指出我正确的方向，那就太好了！谢谢！

编辑 - 对于那些搜索，我使用下面的戈登答案进行以下查询，现在可以使用：

SELECT
    mainTable.cpda_id,
    (CASE 
        WHEN table4.cpda_meta_val = 1 THEN table3.cpda_meta_val 
        ELSE table2.cpda_meta_val 
    END) as override_val_0 
FROM
    cp_data AS mainTable 
LEFT OUTER JOIN
    cp_data AS table0 
        ON table0.cpda_meta_id = mainTable.cpda_id 
        AND table0.cpda_meta_key = '47248' 
LEFT OUTER JOIN
    cp_data AS table1 
        ON table1.cpda_meta_id = mainTable.cpda_id 
        AND table1.cpda_meta_key = '47241' 
LEFT OUTER JOIN
    cp_data AS table2 
        ON table2.cpda_meta_id = mainTable.cpda_id 
        AND table2.cpda_meta_key = '47242' 
LEFT OUTER JOIN
    cp_data AS table3 
        ON table3.cpda_meta_id = mainTable.cpda_id 
        AND table3.cpda_meta_key = '66838' 
LEFT OUTER JOIN
    cp_data AS table4 
        ON table4.cpda_meta_id = mainTable.cpda_id 
        AND table4.cpda_meta_key = '66843' 
LEFT OUTER JOIN
    cp_data AS table5 
        ON table5.cpda_meta_id = mainTable.cpda_id 
        AND table5.cpda_meta_key = '47252' 
LEFT OUTER JOIN
    cp_data AS table6 
        ON table6.cpda_meta_id = mainTable.cpda_id 
        AND table6.cpda_meta_key = '47255' 
WHERE
    mainTable.cpda_meta_id = 47236 
    AND mainTable.cpda_active = 1 
    AND mainTable.cpda_meta_key = 'set_element_stub' 
    AND table0.cpda_meta_val = 49297 
    AND table1.cpda_meta_val != 66116 
    AND (
        table5.cpda_meta_val = 497 
        OR table6.cpda_meta_val = 497
    ) 
ORDER BY
    override_val_0,
    mainTable.cpda_seq,
    mainTable.cpda_id

Answer 1

case语句属于select子句：

SELECT mainTable.cpda_id,
       (CASE WHEN table4.cpda_meta_val = 1 THEN table3.cpda_meta_val
             ELSE table2.cpda_meta_val 
        END) as cpda_meta_val

您也可以将此表达式直接放在order by中（这似乎是您使用table2_cpda_meta_val的唯一地方：

ORDER BY (CASE WHEN table4.cpda_meta_val = 1 THEN table3.cpda_meta_val
                 ELSE table2.cpda_meta_val 
          END),
         mainTable.cpda_seq,
         mainTable.cpda_id

在复杂的连接MySQL查询上覆盖字段

1 个答案: