如何在SQL SELECT语句中执行IF…THEN?

例如:

SELECT IF(Obsolete = 'N' OR InStock = 'Y' ? 1 : 0) AS Saleable, * FROM Product

当前回答

如果您是第一次将结果插入表中,而不是将结果从一个表传输到另一个表,这在Oracle 11.2g中适用:

INSERT INTO customers (last_name, first_name, city)
    SELECT 'Doe', 'John', 'Chicago' FROM dual
    WHERE NOT EXISTS
        (SELECT '1' from customers
            where last_name = 'Doe'
            and first_name = 'John'
            and city = 'Chicago');

其他回答

SQL Server 2012中添加了一个新功能IIF(我们可以简单使用):

SELECT IIF ( (Obsolete = 'N' OR InStock = 'Y'), 1, 0) AS Saleable, * FROM Product

对于使用SQL Server 2012的用户来说,IIF是一个已添加的功能,可以作为Case语句的替代。

SELECT IIF(Obsolete = 'N' OR InStock = 'Y', 1, 0) AS Salable, *
FROM   Product 
SELECT CASE WHEN Obsolete = 'N' or InStock = 'Y' THEN 1 ELSE 0 
             END AS Saleable, * 
FROM Product
 SELECT
   CASE 
      WHEN OBSOLETE = 'N' or InStock = 'Y' THEN 'TRUE' 
      ELSE 'FALSE' 
   END AS Salable,
   * 
FROM PRODUCT

在这种情况下,案例陈述是您的朋友,采取以下两种形式之一:

简单的例子:

SELECT CASE <variable> WHEN <value>      THEN <returnvalue>
                       WHEN <othervalue> THEN <returnthis>
                                         ELSE <returndefaultcase>
       END AS <newcolumnname>
FROM <table>

扩展情况:

SELECT CASE WHEN <test>      THEN <returnvalue>
            WHEN <othertest> THEN <returnthis>
                             ELSE <returndefaultcase>
       END AS <newcolumnname>
FROM <table>

您甚至可以将case语句放在一个order by子句中进行真正的花式排序。