一张图看懂 SQL 的各种 join 用法
作者 | C.L. Moffatt
来源 | codeproject
导读：下图展示了 LEFT JOIN、RIGHT JOIN、INNER JOIN、OUTER JOIN 相关的 7 种用法。
www.codeproject.com/Articles/33052/Visual-Representation-of-SQL-Joins
具体分解如下：
1.INNER JOIN（内连接）
SELECT <select_list>
FROM Table_A A
INNER JOIN Table_B B
ON A.Key = B.Key
2.LEFT JOIN（左连接）
SELECT <select_list>
FROM Table_A A
LEFT JOIN Table_B B
ON A.Key = B.Key
3.RIGHT JOIN（右连接）
SELECT <select_list>
FROM Table_A A
RIGHT JOIN Table_B B
ON A.Key = B.Key
4.OUTER JOIN（外连接）
SELECT <select_list>
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.Key = B.Key
5.LEFT JOIN EXCLUDING INNER JOIN（左连接-内连接）
SELECT <select_list>
FROM Table_A A
LEFT JOIN Table_B B
ON A.Key = B.Key
WHERE B.Key IS NULL
6.RIGHT JOIN EXCLUDING INNER JOIN（右连接-内连接）
SELECT <select_list>
FROM Table_A A
RIGHT JOIN Table_B B
ON A.Key = B.Key
WHERE A.Key IS NULL
7.OUTER JOIN EXCLUDING INNER JOIN（外连接-内连接）
SELECT <select_list>
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.Key = B.Key
WHERE A.Key IS NULL OR B.Key IS NULL
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