join用于将多个表的数据组合显示。
The result of a left outer join (or simply left join) for tables A and B always contains all rows of the "left" table (A), even if the join-condition does not find any matching row in the "right" table (B). This means that if the ON clause matches 0 (zero) rows in B (for a given row in A), the join will still return a row in the result (for that row)—but with NULL in each column from B.
left_join是以左表为主表，join的表是为了配合左表，查不到的部分在左表中就显示为null。
215px-SQL_Join_-_01_A_Left_Join_B.svg.png
参考:
https://coolshell.cn/articles/3463.html
https://en.wikipedia.org/wiki/Join_(SQL)
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