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问题描述
我有一个简单的查询:
SELECT u_name AS user_name FROM users WHERE user_name = "john";
我得到Unknown Column 'user_name' in where clause.即使在select 'u_name as user_name'之后，我也不能在语句的其他部分中引用'user_name'吗?
I get Unknown Column 'user_name' in where clause. Can I not refer to 'user_name' in other parts of the statement even after select 'u_name as user_name'?
推荐答案
SQL从右到左向后评估.因此，where子句将在select子句之前进行解析和评估.因此，尚未发生u_name到user_name的别名.
SQL is evaluated backwards, from right to left. So the where clause is parsed and evaluate prior to the select clause. Because of this the aliasing of u_name to user_name has not yet occurred.
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