示意图
1、内连接（INNER JOIN）
代码如下：
SELECT A.PK AS A_PK,A.Value AS A_Value,B.PK AS B_PK,B.Value AS B_Value
FROM table_a A
INNER JOIN table_b B
ON A.PK = B.PK;
查询结果：
2、左连接（LEFT JOIN）
代码如下：
SELECT A.PK AS A_PK,A.Value AS A_Value,B.PK AS B_PK,B.Value AS B_Value
FROM table_a A
LEFT JOIN table_b B
ON A.PK = B.PK;
查询结果：
3、右连接（RIGHT JOIN）
代码如下：
SELECT A.PK AS A_PK,A.Value AS A_Value,B.PK AS B_PK,B.Value AS B_Value
FROM table_a A
RIGHT JOIN table_b B
ON A.PK = B.PK;
查询结果：
4、全连接（FULL JOIN）
代码如下：
SELECT A.PK AS A_PK,A.Value AS A_Value,B.PK AS B_PK,B.Value AS B_Value
FROM table_a A
FULL JOIN table_b B
ON A.PK = B.PK;
上面代码在mysql执行是报错的，因为mysql不支持全连接，只能用以下代码实现效果，含义是左连接+右连接+去重=全。
所谓的去重，实际上就是使用UNION，取并集：
SELECT A.PK AS A_PK,A.Value AS A_Value,B.PK AS B_PK,B.Value AS B_Value
FROM table_a A
LEFT JOIN table_b B
ON A.PK = B.PK
UNION
SELECT A.PK AS A_PK,A.Value AS A_Value,B.PK AS B_PK,B.Value AS B_Value
FROM table_a A
RIGHT JOIN table_b B
ON A.PK = B.PK;
查询结果：
5、左连接不包含内连接
代码如下：
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
LEFT JOIN Table_B B
ON A.PK = B.PK
WHERE B.PK IS NULL
查询结果：
6、右连接不包含内连接
代码如下：
SELECT A.PK AS A_PK, A.Value AS A_Value, B.PK AS B_PK,
B.Value AS B_Value
FROM Table_A A
RIGHT JOIN Table_B B
ON A.PK = B.PK
WHERE A.PK IS NULL
查询结果：
7、外连接不包含内连接
代码如下：
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.PK = B.PK
WHERE A.PK IS NULL
OR B.PK IS NULL
由于mysql不支持full join，只能通过下面代码模拟实现：
SELECT A.PK AS A_PK, A.Value AS A_Value, B.PK AS B_PK,
B.Value AS B_Value
FROM table_a A
LEFT JOIN table_b B
ON A.PK = B.PK
WHERE B.PK IS NULL
UNION ALL
SELECT *
FROM table_a A
RIGHT JOIN table_b B
ON A.PK = B.PK
WHERE A.PK IS NULL;
查询结果：
