LEFT JOIN别名的未知列错误

Question

我试图在左连接中添加别名，以便我可以在同一个表（不同的键）上进行两次连接，但是遇到了这个问题。我已经将问题简化为问题所在，以保持简单。

SELECT stock_items.id AS stockid, exchange_rates.rate AS rate
FROM stock_items
LEFT JOIN exchange_rates AS exr ON stock_items.currency = exchange_rates.id

当我运行此查询时，我得到了

1054 - 'on clause'

中的未知列'exchange_rates.id'

但是，如果我删除了别名，那么它就像这样...

SELECT stock_items.id AS stockid, exchange_rates.rate AS rate 
FROM stock_items 
LEFT JOIN exchange_rates ON stock_items.currency = exchange_rates.id

此查询运行正常......？

Answer 1

如果您添加alias，则应在join。

中使用它

SELECT stock_items.id AS stockid, exr.rate AS rate
FROM stock_items
LEFT JOIN exchange_rates exr ON stock_items.currency = exr.id