Question

我正在尝试在mysql DB上使用select case语句。

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错误： ......正确的语法在'AS small，CASE（Amount＆gt; 0）

附近使用

我想要我的输出

SELECT 
t.name, t.colour,
CASE WHEN (Amount < 0) AS small,  CASE WHEN (Amount > 0) AS large FROM t

一直没有运气。请帮助。

Answer 1

这是：

char

了解SELECT t.name, t.colour, CASE WHEN (t.Amount < 0) THEN t.Amount END AS small, CASE WHEN (t.Amount > 0) THEN t.Amount END AS large FROM t：

示例＃1：

假设，

CASE WHEN

让我们使用IF n < 0 THEN 'N is negative' else if n == 0 THEN 'N is Zero' else 'N is positive'表达式在MySQL中转换此if-else if链：

CASE WHEN

<强>输出：

SET @n := -9;
SELECT
CASE WHEN @n <0 THEN 'N is Negative'
     WHEN @n =0 THEN 'N is 0'
     ELSE 'N is positive' END AS output;

示例＃2：

现在我们要在MySQL中转换以下三个if语句

    output
N is Negative

<强>输出：

现在输出有三列：

IF n <0 THEN 'N is Negative'
IF n == 0 THEN 'N is Zero'
IF n > 0 THEN 'N is Positive'

SET @n := 5;
SELECT
    CASE WHEN @n <0 THEN 'N is Negative' END AS negativeOutput,
    CASE WHEN @n =0 THEN 'N is 0' END AS zeroOutput,
    CASE WHEN @n > 0 THEN 'N is positive' END AS positiveOutput;

Answer 2

您只能为整个表达式设置别名，而不能为表达式的各个组件设置别名：

predict()

如何使用Mysql SELECT CASE WHEN表达式

2 个答案: