返回视图中的两列之一 - 无论哪一列不为空 [英] Return one of two columns in a view - whichever one is not null

问题描述

我有一个包含三列的表格:

I have a table with three columns:

ColumnA          ColumnB         ColumnC
AAA               NULL            123
BBB               222             NULL
CCC               NULL            NULL

我想创建一个 SELECT 语句，它将返回 ColumnA，然后是第二列，除非 ColumnB 为空，否则它将显示 ColumnB 的值；否则它会显示 ColumnC 的值，即使它是 NULL.我可以为此使用 IF 语句吗?类似的东西:

I would like to create a SELECT statement which will return ColumnA, and then a second column which will either show the value of ColumnB unless ColumnB is null; otherwise it will show the value of ColumnC, even it it's NULL. Can I use an IF statement for that? Something like:

SELECT ColumnA, 
IF(ColumnB IS NULL, ColumnC, ColumnB)
FROM table

**如果我让它工作，下一步将返回连接列的值而不是 ColumnB.实际上，IF 语句将是

**If I get this working, the next step would be to return the value of a joined column instead of ColumnB. In effect the IF statement would be

IF(table.ColumnB IS NULL, table.ColumnC, table2.ColumnD)

推荐答案

阅读到问题的结尾，听起来您需要使用 CASE

Reading to the end of your question it sounds like you need to use CASE

CASE WHEN table.ColumnB IS NULL 
     THEN table.ColumnC 
     ELSE table2.ColumnD 
END AS Whatever

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