选择查询中的 MySQLIFELSEIF

我试图根据用户选择的数量来选择产品的不同价格。 这是我正在处理的查询(它有一个语法错误) :

 select id,
(SELECT
IF(qty_1<='23',price,1)
ELSEIF(('23'>qty_1 && qty_2<='23'),price_2,1)
ELSEIF(('23'>qty_2 && qty_3<='23'),price_3,1)
ELSEIF('23'>qty_3,price_4,1)
END IF) as total
from product;
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IF() in MySQL is a ternary function, not a control structure -- if the condition in the first argument is true, it returns the second argument; otherwise, it returns the third argument. There is no corresponding ELSEIF() function or END IF keyword.

The closest equivalent to what you've got would be something like:

IF(qty_1<='23', price,
IF('23'>qty_1 && qty_2<='23', price_2,
IF('23'>qty_2 && qty_3<='23', price_3,
IF('23'>qty_3, price_4, 1)
)
)
)

The conditions don't all make sense to me (it looks as though some of them may be inadvertently reversed?), but without knowing what exactly you're trying to accomplish, it's hard for me to fix that.

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最佳答案

You have what you have used in stored procedures like this for reference, but they are not intended to be used as you have now. You can use IF as shown by duskwuff. But a Case statement is better for eyes. Like this:

select id,
(
CASE
WHEN qty_1 <= '23' THEN price
WHEN '23' > qty_1 && qty_2 <= '23' THEN price_2
WHEN '23' > qty_2 && qty_3 <= '23' THEN price_3
WHEN '23' > qty_3 THEN price_4
ELSE 1
END) AS total
from product;

This looks cleaner. I suppose you do not require the inner SELECT anyway..

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I found a bug in MySQL 5.1.72 when using the nested if() functions .... the value of column variables (e.g. qty_1) is blank inside the second if(), rendering it useless. Use the following construct instead:

case
when qty_1<='23' then price
when '23'>qty_1 && qty_2<='23' then price_2
when '23'>qty_2 && qty_3<='23' then price_3
when '23'>qty_3 then price_4
else 1
end
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As per Nawfal's answer, IF statements need to be in a procedure. I found this post that shows a brilliant example of using your script in a procedure while still developing and testing. Basically, you create, call then drop the procedure:

https://gist.github.com/jeremyjarrell/6083251

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For your question : 

SELECT id,
IF(qty_1 <= '23', price,
IF(('23' > qty_1 && qty_2 <= '23'), price_2,
IF(('23' > qty_2 && qty_3 <= '23'), price_3,
IF(('23' > qty_2 && qty_3<='23'), price_3,
IF('23' > qty_3, price_4, 1))))) as total
FROM product;

You can use the if - else control structure or the IF function in MySQL.

Reference:
http://easysolutionweb.com/sql-pl-sql/how-to-use-if-and-else-in-mysql/


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