LEFT JOIN Alias的未知列错误

Question

I was trying to add aliases to my left joins so that I could do two joins on the same table (different key) but ran in to this problem. I've stripped down the query to where the problem is to keep things simple.

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

When I run this query I get

1054 - Unknown column 'exchange_rates.id' in 'on clause'

However if I remove the alias so it looks like this...

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

This query runs fine... ?

Answer 1

if you add an alias , you should use it in your join .

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