A LEFT JOIN B ON C UNION A RIGHT JOIN B ON C :将A与B按照条件C进行全外连接//两表内容都有可能会有空
7种JOIN的实现
//内连接(A且B)
SELECT<select_list>
FROM TableA A
JOIN TableB B
ON A.Key=B.Key
//左外连接(A)
SELECT<select_list>
FROM TableA A
LEFT JOIN TableB B
ON A.Key=B.Key
//右外连接(B)
SELECT<select_list>
FROM TableA A
RIGHT JOIN TableB B
ON A.Key=B.Key
//A且非B
SELECT<select_list>
FROM TableA A
LEFT JOIN TableB B
ON A.Key=B.Key
WHERE B.Key IS NULL
//B且非A
SELECT<select_list>
FROM TableA A
RIGHT JOIN TableB B
ON A.Key=B.Key
WHERE A.Key IS NULL
//全外连接(A并B)
SELECT<select_list>
FROM TableA A
Union TableB B
ON A.Key=B.Key
//A并B且非A交B
SELECT<select_list>
FROM TableA A
LEFT JOIN TableB B
ON A.Key=B.Key
WHERE B.Key IS NULL
Union ALL
SELECT<select_list>
FROM TableA A
RIGHT JOIN TableB B
ON A.Key=B.Key
WHERE A.Key IS NULL