Question

我在单独的查询中查询两个表。这样做可以在不到一秒的时间内得到正确的结果，但是，如果我尝试合并它们，我甚至需要杀死查询（很多记录）需要很长时间。我想知道我做错了什么。

所需输出

+--------+-----------+-----------+-----------+    
| name   | computers | printers  |  tvs      |
+--------+-----------+-----------+-----------+
| Peter  |  12       |   1       |   5       |
| Josh   |  20       |   4       |           |
| Bob    |  15       |   4       |   9       |
+--------+-----------+-----------+-----------+

查询1

SELECT
    names.name,
    SUM(IF(pcs.electronic = 'computers', 1,0)) AS computers,
    SUM(IF(pcs.electronic = 'printers', 1,0)) AS printers
FROM
    electronics.pcs LEFT JOIN electronics.orders 
    ON pcs.id = orders.id
    LEFT JOIN electronics.buying
    ON orders.id = buying.id
    LEFT JOIN dbnames.names names
    ON names.clientid = pcs.clientid
WHERE
    pcs.clientid IS NOT NULL AND
    pcs.belongs IN (10)
GROUP BY pcs.clientid

表1 - 查询1

+--------+-----------+-----------+ 
| name   | computers | printers  | 
+--------+-----------+-----------+
| Peter  |  12       |   1       | 
| Josh   |  20       |   4       |
| Bob    |  15       |   4       |
+--------+-----------+-----------+

查询2

SELECT
    names.name,
    SUM(IF(tvs.electronic = 'tvs', 1,0)) AS tvs
FROM
    electronics.tvs LEFT JOIN dbnames.names names
    ON names.clientid = tvs.clientid
WHERE
    tvs.belongs IN (10)
GROUP BY tvs.clientid

表2 - 查询2

+--------+-----------+
| name   |  tvs      |
+--------+-----------+
| Peter  |   5       |
| Bob    |   9       |
+--------+-----------+

Answer 1

为什么不做一个简单的 UNION ？

SELECT
    names.name,
    SUM(IF(pcs.electronic = 'computers', 1,0)) AS computers,
    SUM(IF(pcs.electronic = 'printers', 1,0)) AS printers
FROM
    electronics.pcs LEFT JOIN electronics.orders 
    ON pcs.id = orders.id
    LEFT JOIN electronics.buying
    ON orders.id = buying.id
    LEFT JOIN dbnames.names names
    ON names.clientid = pcs.clientid
WHERE
    pcs.clientid IS NOT NULL AND
    pcs.belongs IN (10)
GROUP BY pcs.clientid

UNION ALL

SELECT
    names.name,
    SUM(IF(tvs.electronic = 'tvs', 1,0)) AS tvs
FROM
    electronics.tvs LEFT JOIN dbnames.names names
    ON names.clientid = tvs.clientid
WHERE
    tvs.belongs IN (10)
GROUP BY tvs.clientid

只是一个想法...

Answer 2

完全连接可以很好地工作，但由于MySQL不支持完全连接..你可以在两者之间使用UNION ALL并从相应的列中获取MAX。

SELECT clientid, name,
    MAX(computers) computers,
    MAX(printers) printers,
    MAX(tvs) tvs
FROM
(
    SELECT
        pcs.clientid,
        names.name,
        SUM(IF(pcs.electronic = 'computers', 1,0)) AS computers,
        SUM(IF(pcs.electronic = 'printers', 1,0)) AS printers,
        NULL
    FROM
        electronics.pcs LEFT JOIN electronics.orders 
        ON pcs.id = orders.id
        LEFT JOIN electronics.buying
        ON orders.id = buying.id
        LEFT JOIN dbnames.names names
        ON names.clientid = pcs.clientid
    WHERE
        pcs.clientid IS NOT NULL AND
        pcs.belongs IN (10)
    GROUP BY pcs.clientid, names.name
    UNION ALL
    SELECT
        tvs.clientid,
        names.name,
        NULL, NULL,
        SUM(IF(tvs.electronic = 'tvs', 1,0)) AS tvs
    FROM
        electronics.tvs LEFT JOIN dbnames.names names
        ON names.clientid = tvs.clientid
    WHERE
        tvs.belongs IN (10)
    GROUP BY tvs.clientid, names.name
) Q2
GROUP BY clientid, name

Answer 3

select *
from
(
  ... first query
) as query1
left join
(
  ... second query
) as query2
on query1.name=query2.name;

组合表时MySQL查询错误

3 个答案: