0

可能重复:
不使用 select * 的原因是什么?

你能解释一下select_expr语句中的哪个子句SELECT有好处吗?我一直这样*select_expr

SELECT * FROM table_name WHERE ...

这已经足够了。

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3 回答 3

1

一个比其他例子更重要的例子:

  • 您需要使用聚合函数:

    SELECT ID, COUNT(*) AS Num_Total
      FROM OrderItem
     GROUP BY OrderID;
    
  • 您需要从 UNION 查询的两个部分获取信息。我最近在回答另一个问题时使用了它:

    SELECT v1.ID AS OrderID_1, v1.NA AS NS, v2.ID AS OrderID_2, v2.NB AS NL, v3.NC AS NC
      FROM (SELECT ID, COUNT(*) AS NA
              FROM OrderItem
             WHERE OrderID = <specified order ID>
             GROUP BY OrderID
           ) AS v1
      JOIN (SELECT ID, COUNT(*) AS NB
              FROM OrderItem
             WHERE OrderID != <specified order ID>
             GROUP BY OrderID
           ) AS v2
        ON v1.NA <= v2.NB
      JOIN (SELECT I1.ID, COUNT(*) AS NC
              FROM OrderItem AS I1
              JOIN OrderItem AS I2 ON I2.ItemID = I1.ItemID AND I2.OrderID = <specified order ID>
             WHERE I1.OrderID != <specified order ID>
             GROUP BY I1.ID
           ) AS v3
        ON v3.ID = v2.ID
    UNION
    SELECT v2.ID AS OrderID_1, v2.NB AS NS, v1.ID AS OrderID_2, v2.NA AS NL, v3.NC AS NC
      FROM (SELECT ID, COUNT(*) AS NA
              FROM OrderItem
             WHERE OrderID = <specified order ID>
             GROUP BY OrderID
           ) AS v1
      JOIN (SELECT ID, COUNT(*) AS NB
              FROM OrderItem
             WHERE OrderID != <specified order ID>
             GROUP BY OrderID
           ) AS v2
        ON v1.NA > v2.NB
      JOIN (SELECT I1.ID, COUNT(*) AS NC
              FROM OrderItem AS I1
              JOIN OrderItem AS I2 ON I2.ItemID = I1.ItemID AND I2.OrderID = <specified order ID>
             WHERE I1.OrderID != <specified order ID>
             GROUP BY I1.ID
           ) AS v3
        ON v3.ID = v1.ID
    

在该查询中,QuantityOrderItem 表中的列与我正在执行的计算没有任何关系。如果不使用“select_expr”而不是“SELECT *”,就无法编写该查询。您可能会注意到,最初的“聚合函数”建议是这个较大查询中的子查询之一(它出现了两次——您相信优化器会准确优化)。

于 2012-11-19T00:01:14.497 回答
1

有几个原因:

  • 您可以添加计算列:select a, b, a+b FROM table
  • 您可以限制传输的数据量。
  • 您可以为各个列指定一个新名称。
  • 您可以更安全、更高效地进行编程。

补充第一点和第二点:特别是通常需要列限制。

添加到第四点:在进行临时查询时,我几乎总是使用*. 但是程序应该总是枚举列。这可以防止令人惊讶的列重新排序,并且还可以明确实际使用了哪些数据。

于 2012-11-18T22:23:17.360 回答
0

想象一下,您的表有一百万列,您只对其中一个感兴趣,并且您正在通过网络通过 30 波特连接与您的数据库通信。

于 2012-11-18T22:16:41.500 回答