Question

可能重复：
What is the reason not to use select *?

您能否解释一下select_expr语句中的SELECT子句有用吗？我一直在使用* select_expr这样

SELECT * FROM table_name WHERE ...

就够了。

Answer 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。
您可以为各列添加新名称。
您可以更安全，更高效地进行编程。

添加到第一点和第二点：特别是通常需要列限制。

添加到第四点：在进行adhoc查询时，我几乎总是使用*。但程序应始终枚举列。这可以防止令人惊讶的列重新排序，并且还可以清楚地显示实际使用的数据。

Answer 2

除了已经建议的其他例子之外的一个例子：

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。我最近在另一个问题answer中使用了这个问题：

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

该查询中没有任何一个是OrderItem表中的Quantity列，与我正在进行的计算有任何相关性。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，初始“聚合函数”建议是此较大查询中的子查询之一（并且它出现两次 - 您相信优化程序将准确地进行优化）。

Answer 3

想象一下，您的表有一百万列，您只对一个列感兴趣，并且您通过30波特连接通过网络与数据库通信。

SELECT语句中的select_expr子句有用吗？

3 个答案: