带有某些 IF ELSE If ELSE 条件的 Mysql 查询

Question

我正在尝试编写带有某些 if 条件的 mysql 查询。

我的数据库结构是

货币(表)

++++++++++++++++++++++++++
id | currency | rate
++++++++++++++++++++++++++
1      USD       1
2      INR       67.10043
3      GBP       0.761203
4      EUR       0.89295
++++++++++++++++++++++++++

com_payments (Table)
++++++++++++++++++++++++++
id | amt_curr | amt_paid
++++++++++++++++++++++++++
1      EUR        300.89
2      GBP        390.90
3      USD        600.00
++++++++++++++++++++++++++

我正在寻找的查询是 if

if (amt_curr=='USD') {
    $totalinr = (amt_paid * rate); (rate it should take from currency table where currency='USD' from currency table)
} else if ($currency1=='GBP'){
    totalinr = ((select currency, rate from Currency where currency=INR / 0.761203 (GBP value from currency table)) * amt_paid);
} else if ($currency1=='EUR'){
    totalinr = ((select currency, rate from Currency where currency=INR / 0.89295 (EUR value from currency table)) * amt_paid); 
}

我试图用 mysql 查询来实现这一点:我尝试了类似的方法，但它不起作用:

CASE WHEN (amt_curr=='USD') THEN (amt_paid * rate) (rate it should take from currency table where currency='USD' from currency table)
     WHEN ($currency1=='GBP') THEN ((select currency, rate from Currency where currency=INR / 0.761203 (GBP value from currency table)) * amt_paid) 
     WHEN ($currency1=='EUR') THEN ((select currency, rate from Currency where currency=INR / 0.89295 (EUR value from currency table)) * amt_paid)
END AS totalinr

Answer 1

这可以简单地通过连接来完成，您不需要使用CASE EXPRESSION:

SELECT t.*,
       t.amt_paid /  c.rate
FROM com_payments t
INNER JOIN currency c
 ON(c.currency = p.amt_currency)

编辑:

SELECT t.*,
       CASE WHEN c.currency = 'USD' THEN t.amt_paid * c.rate --By the way, you can use only amt_paid since rate is always 1
                                    ELSE t.amt_paid / c.rate
       END
FROM com_payments t
INNER JOIN currency c
 ON(c.currency = p.amt_currency)