执行查询的最有效方式

Question

我的目标是从外键值与另一个表的子查询返回的 ID 匹配的表中获取所有记录。

我尝试了几种组合，但它们甚至都无法编译。

为了澄清，请考虑以下查询：

SELECT *
FROM   `news`
WHERE  IDFIRM IN (SELECT ID FROM firm WHERE Block=0)
AND    Actual=1

这是一个简单的查询，但除了这个子查询中的ID，我还需要检索其他列并返回它们作为响应。

类似这样的东西（但当然这是行不通的）：

SELECT news.*, sub.name
FROM   `news`,
       (SELECT * FROM firm WHERE Block=0) AS sub
WHERE  news.IDFIRM IN (SELECT sub.ID FROM sub)
AND    news.Actual=1

我知道如何使用连接来实现这一点，但问题是可以有多个 IN 语句（来自不同的表），如下所示：

SELECT *
FROM   `news`
WHERE  IDFIRM IN (SELECT ID FROM firm WHERE Block=0)
AND    Actual=1
AND    id_publisher IN (SELECT ID FROM publisher WHERE Block=0)

因此，根据请求的不同，此查询可能会变得非常不同。

如何创建最有效的查询来解决此任务？或者只有多个查询才有可能？

谢谢。

Answer 1

您已经在建议加入。 你可以使用多个 JOIN 来得到你想要的。

SELECT * FROM news
JOIN firm ON IDFIRM = firm.ID
WHERE Block = 0
JOIN publisher ON id_publisher = publisher.ID
WHERE Block = 0

我是凭记忆这样做的，但我认为应该让你接近。

Answer 2

假设您的索引在JOIN 的两侧都正常，它们实际上会被利用，而子查询可能使用一侧。

SELECT [column_list]
FROM news n
  INNER JOIN firm f
    ON n.idfirm = f.id
  INNER JOIN publisher p
    ON n.idpublisher = p.id
WHERE n.actual = 1
  AND f.block = 0
  AND p.block = 0

Answer 3

您绝对应该使用JOIN 而不是IN (SELECT ...)：

查询 1：

SELECT *
FROM   `news`
WHERE  IDFIRM IN (SELECT ID FROM firm WHERE Block=0)
AND    Actual=1

替换为：

SELECT     news.*
FROM       news
INNER JOIN form ON news.IDFIRM = form.ID
WHERE      news.Actual=1

查询 2：

SELECT news.*, sub.name
FROM   `news`,
       (SELECT * FROM firm WHERE Block=0) AS sub
WHERE  news.IDFIRM IN (SELECT sub.ID FROM sub)
AND    news.Actual=1

替换为：

SELECT     news.*,
           firm.name
FROM       news
INNER JOIN firm ON news.IDFIRM = firm.ID 
WHERE      news.Actual=1
AND        firm.Block=0

查询 3：

SELECT *
FROM   `news`
WHERE  IDFIRM IN (SELECT ID FROM firm WHERE Block=0)
AND    Actual=1
AND    id_publisher IN (SELECT ID FROM publisher WHERE Block=0)

替换为：

SELECT     news.*
FROM       news
INNER JOIN firm      ON news.IDFIRM = firm.ID
INNER JOIN publisher ON news.id_publisher = publisher.ID
WHERE      news.Actual=1
AND        firm.Block=0
AND        publisher.Block=0

添加表格和条件

所以想法是你添加所有你需要的表，像这样的子句：

INNER JOIN table1 ON news.<foreign-key> = table1.ID

并且您在WHEREclause 中添加所有条件，如下所示：

WHERE   table1.field = <literal>
AND     table2.field = <other literal>
AND     ...