Question

我有下表：

TestTable  
  ColumnA  
  Column1  
  Column2  
  Column3  
  Column4

我想编写一个简单的SQL语句来检查ColumnA，并根据它是否为null返回某些列。

像（伪）的东西：

If ColumnA Is Null
  SELECT ColumnA, Column1, Column2 
  FROM TestTable
Else
  SELECT ColumnA, Column3, Column4
  FROM TestTable

有什么想法吗？

Answer 1

使用SQL CASE表达式：

SELECT
    ColumnA,
    CASE WHEN ColumnA IS NULL THEN Column1 ELSE Column3 END AS ColumnB,
    CASE WHEN ColumnA IS NULL THEN Column2 ELSE Column4 END AS ColumnC
FROM
    TestTable

请参阅：
CASE (SQL-Server Transact-SQL)
CASE Expressions (Oracle)

Answer 2

我能想到几条路线，没有一条路可能很漂亮......

人们经常会找到第一个，然后寻找更好的东西。我不确定那里有什么更好的。

SELECT
  ColumnA,
  CASE WHEN ColumnA IS NULL THEN Column1 ELSE Column2 END AS ColumnB,
  CASE WHEN ColumnA IS NULL THEN Column3 ELSE Column4 END AS ColumnC
FROM
  yourTable

或者...

SELECT
  yourTable.ColumnA,
  subTable.ColumnB,
  subTable.ColumnC
FROM
  yourTable
CROSS APPLY
(
  SELECT yourTable.Column1 AS ColumnB, yourTable.Column3 AS ColumnC WHERE yourTable.ColumnA IS NULL
  UNION ALL
  SELECT yourTable.Column2 AS ColumnB, yourTable.Column4 AS ColumnC WHERE yourTable.ColumnA IS NOT NULL
)
  AS subTable

或者...

SELECT
  ColumnA,
  Column1 AS ColumnB,
  Column2 AS ColumnC
FROM
  yourTable
WHERE
  ColumnA IS NULL

UNION ALL

SELECT
  ColumnA,
  Column2 AS ColumnB,
  Column4 AS ColumnC
FROM
  yourTable
WHERE
  ColumnA IS NOT NULL

Answer 3

另一种给这只特殊猫皮肤涂抹的方法：

SELECT ColumnA,
       NVL2(ColumnA, Column3, Column1),
       NVL2(ColumnA, Column4, Column2)
  FROM TestTable

分享并享受。

如果field为null，则拉出某些字段;否则，拉其他领域

3 个答案: