SELECT 语句中的 select_expr 子句适用于什么？ [复制]

Question

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

您能解释一下SELECT 语句中的select_expr 子句有什么用吗？我一直在使用* 为select_expr 像这样

SELECT * FROM table_name WHERE ...

这就够了。

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。
• 您可以为各个列指定一个新名称。
• 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。

补充第四点：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，并清楚地表明实际使用了哪些数据。

Answer 2

一个超越其他已经建议的例子：

• 你需要使用聚合函数：

  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answer 中使用了这个来回答另一个问题：

  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
  

在该查询中，OrderItem 表中的Quantity 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）。

Answer 3

假设您的表有一百万列，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。