JOIN(SELECT ...)ue ON 1 = 1? [英] JOIN (SELECT ... ) ue ON 1=1?
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问题描述
我正在Redshift中读取SQL查询，但不了解最后一部分:
I am reading an SQL query in Redshift and can't understand the last part:
...
LEFT JOIN (SELECT MIN(modified) AS first_modified FROM user) ue
ON 1=1
ON 1=1在这里是什么意思?
推荐答案
它只是在进行交叉连接，它选择了第一张表中的所有行和第二张表中的所有行，并显示为笛卡尔积，即具有所有可能.
It's simply doing a cross join, which selects all rows from the first table and all rows from the second table and shows as cartesian product, i.e. with all possibilities.
JOIN(向左，向内，向右等)语句通常需要'ON ...'条件.放入1 = 1就像说"1 = 1始终为真，什么也不要消除".
JOIN (LEFT, INNER, RIGHT, etc.) statements normally require an 'ON ..." condition. Putting in 1=1 is like saying "1=1 is always true, do don't eliminate anything".
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