版權聲明：轉載請註明作者（獨孤尚良dugushangliang）出處：https://blog.csdn.net/dugushangliang/article/details/103817246
搜了搜，基本上都是寫內連接、外連接、交叉連接這三種類型，但我發現PostgreSQL還有自連接。不妨一併寫來做個記錄。
先說概念：
內連接，就是兩個表逐行匹配，匹配上的內容都顯示，沒有匹配的都不顯示。
外連接有三種，左外連接，右外連接，全外連接。
左外連接是以左表爲基礎，左表內容全部顯示，右表有匹配到左表的則顯示，否則不顯示。
右外連接是以右表爲基礎，右表內容全部顯示，左表有匹配到右表的則顯示，否則不顯示。
全外連接是以兩表爲基礎，顯示三部分內容，一部分是內連接的內容，即兩表匹配的內容，一部分是左表有而右表無的，一部分是左表無右表有的。
自連接是逐行，用當前這行數據和這個表中其他行進行匹配。
交叉連接最省事，笛卡爾積，左表m行右表n行，則結果是m*n行。
下面展示具體例子來幫助理解。
下面是兩個表的內容。
mydb=# select * from weather;
city | temp_lo | temp_hi | prcp | date
---------------+---------+---------+------+------------
San Francisco | 46 | 50 | 0.25 | 1994-11-27
San Francisco | 43 | 57 | 0 | 1994-11-29
Hayward | 37 | 54 | | 1994-11-29
(3 行記錄)
mydb=# select * from cities;
name | location
---------------+-----------
San Francisco | (-194,53)
London | (0,51)
(2 行記錄)
內連接有兩種寫法：
mydb=# SELECT *
mydb-# FROM weather, cities
mydb-# WHERE city = name;
city | temp_lo | temp_hi | prcp | date | name | location
---------------+---------+---------+------+------------+---------------+-----------
San Francisco | 46 | 50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
San Francisco | 43 | 57 | 0 | 1994-11-29 | San Francisco | (-194,53)
(2 行記錄)
mydb=# SELECT *
mydb-# FROM weather INNER JOIN cities ON (weather.city = cities.name);
city | temp_lo | temp_hi | prcp | date | name | location
---------------+---------+---------+------+------------+---------------+-----------
San Francisco | 46 | 50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
San Francisco | 43 | 57 | 0 | 1994-11-29 | San Francisco | (-194,53)
(2 行記錄)
外連接有三種：左外連接，右外連接，全外連接。
mydb=# SELECT *
mydb-# FROM weather LEFT OUTER JOIN cities ON (weather.city = cities.name);
city | temp_lo | temp_hi | prcp | date | name | location
---------------+---------+---------+------+------------+---------------+-----------
San Francisco | 46 | 50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
San Francisco | 43 | 57 | 0 | 1994-11-29 | San Francisco | (-194,53)
Hayward | 37 | 54 | | 1994-11-29 | |
(3 行記錄)
mydb=# select * from weather right outer join cities on(weather.city=cities.name);
city | temp_lo | temp_hi | prcp | date | name | location
---------------+---------+---------+------+------------+---------------+-----------
San Francisco | 43 | 57 | 0 | 1994-11-29 | San Francisco | (-194,53)
San Francisco | 46 | 50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
| | | | | London | (0,51)
(3 行記錄)
mydb=# select * from weather full outer join cities on(weather.city=cities.name);
city | temp_lo | temp_hi | prcp | date | name | location
---------------+---------+---------+------+------------+---------------+-----------
San Francisco | 46 | 50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
San Francisco | 43 | 57 | 0 | 1994-11-29 | San Francisco | (-194,53)
Hayward | 37 | 54 | | 1994-11-29 | |
| | | | | London | (0,51)
(4 行記錄)
表交叉連接：
mydb=# SELECT *
mydb-# FROM weather, cities;
city | temp_lo | temp_hi | prcp | date | name | location
---------------+---------+---------+------+------------+---------------+-----------
San Francisco | 46 | 50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
San Francisco | 46 | 50 | 0.25 | 1994-11-27 | London | (0,51)
San Francisco | 43 | 57 | 0 | 1994-11-29 | San Francisco | (-194,53)
San Francisco | 43 | 57 | 0 | 1994-11-29 | London | (0,51)
Hayward | 37 | 54 | | 1994-11-29 | San Francisco | (-194,53)
Hayward | 37 | 54 | | 1994-11-29 | London | (0,51)
(6 行記錄)
表自連接：
mydb=# SELECT W1.city, W1.temp_lo AS low, W1.temp_hi AS high,
mydb-# W2.city, W2.temp_lo AS low, W2.temp_hi AS high
mydb-# FROM weather W1, weather W2
mydb-# WHERE W1.temp_lo < W2.temp_lo
mydb-# AND W1.temp_hi > W2.temp_hi;
city | low | high | city | low | high
---------------+-----+------+---------------+-----+------
San Francisco | 43 | 57 | San Francisco | 46 | 50
Hayward | 37 | 54 | San Francisco | 46 | 50
(2 行記錄)
獨孤尚良dugushangliang——著
