sql-server-2005 - 在查询中两次使用相同的函数 (SQL Server)

Question

在 SQL Server 2005 中，当我编写一个查询时

SELECT m.*, a.price p1, b.price p2
FROM mytable m
LEFT JOIN products_table_1 a
ON my_hash_function(m.name) = a.hash
LEFT JOIN products_table_2 b
ON my_hash_function(m.name) = b.hash

是 my_hash_function(m.name)计算两次还是只计算一次？如果两次，我如何使用变量来避免这种情况？

Answer 1

select  mm.*, a.price p1, b.price p2 from   
    (SELECT m.*, my_hash_function(m.name) as name
    FROM mytable m) mm
    LEFT JOIN products_table_1 a
    ON mm.name = a.hash
    LEFT JOIN products_table_2 b
    ON mm.name = b.hash