左连接后如何加入复杂的mysql查询

Question

我有这个查询

SELECT 
       currency_code, 
       SUM(CASE WHEN TYPE = 'buy'THEN to_amount END ) AS BUY, 
       SUM(CASE WHEN TYPE = 'sell' THEN to_amount END ) AS SELL, 
       SUM(CASE WHEN TYPE = 'sell' THEN rate END ) AS SELL_RATE, 
       SUM(CASE WHEN TYPE = 'buy' THEN rate END ) AS BUY_RATE, 
       AVG(CASE WHEN TYPE = 'buy' THEN rate END ) AS AVG_BUY_RATE, 
       AVG(CASE WHEN TYPE = 'sell' THEN rate END ) AS AVG_SELL_RATE
FROM tb_currency
LEFT JOIN tb_bill 
   ON tb_currency.CURRENCY_ID = tb_bill.CURRENCY_ID
     AND tb_bill.TYPE  IN ('buy', 'sell')
     AND date( DATE_TIME ) >= '2011-01-01'
     AND date( DATE_TIME ) <= '2011-01-11'
GROUP BY currency_code

将输出：

现在我想将此查询与另一个名为 tb_user

的表联系起来

tb_user的PK名为 user_id ，上面查询中使用的 tb_bill 也有外键名为 user_id

tb_user

user_id (pk)| user_name | branch_id

tb_bill

bill_id (pk) | user_id (fk)|

期望的结果应该是上面的图片加上一列branch_id。 如果它没有branch_id，则返回null。

我尝试过几次但仍无法正确加入。希望你们能帮忙。

感谢。

Answer 1

将user_id添加到SELECT部分 之后

LEFT JOIN tb_bill ON tb_currency.CURRENCY_ID = tb_bill.CURRENCY_ID

地方

LEFT JOIN tb_user ON tb_user.id = tb_bill.user_id

你也错过了WHERE（首先放置AND） 和

GROUP BY currency_code, user_id

Answer 2

连接中的三个条件（AND子句）可能会给您带来麻烦。这三个条件是选择标准，而不是加入标准。

另外，你对CASE的使用对我来说很奇怪。我确信它有效，但IF可能更适合单条件函数。在下面，如果字段是浮点而不是整数，则将0替换为0.0。

SELECT currency_code, 
    SUM(IF(TYPE = 'buy', to_amount, 0)) AS BUY,
    SUM(IF(TYPE = 'sell', to_amount, 0)) AS SELL, 
    SUM(IF(TYPE = 'sell', rate, 0)) AS SELL_RATE,
    SUM(IF(TYPE = 'buy', rate, 0)) AS BUY_RATE,
    AVG(IF(TYPE = 'buy', rate, 0)) AS AVG_BUY_RATE,
    AVG(IF(TYPE = 'sell', rate, 0)) AS AVG_SELL_RATE,
    tb_user.whatever_field,
    tb_user.whatever_other_field
FROM tb_currency
    LEFT JOIN tb_bill ON tb_currency.CURRENCY_ID = tb_bill.CURRENCY_ID
    LEFT JOIN tb_user ON tb_bill.user_id = tb_user.user_id
WHERE tb_bill.TYPE IN ('buy', 'sell')
    AND date( DATE_TIME ) >= '2011-01-01'
    AND date( DATE_TIME ) <= '2011-01-11'
GROUP BY currency_code, tb_user.user_id

最后，全局字段名称看起来也很奇怪。无论什么对你有用。