数据库 发布日期:2024/12/25 浏览次数:1
city表:
create table city(id int,name text); insert into city values(0,'北京'),(1,'西安'),(2,'天津'),(3,'上海'),(4,'哈尔滨'),(5,'西藏')
person表:
create table person(id int,lastname char(20)); insert into person values(0,'Tom'),(2,'Lily'),(3,'Mary'),(5,'Coco'); select * from city;
select * from person;
inner join(等值连接) 只返回两个表中联结字段相等的行
sql语句:
select * from city inner join person on city.id = person.id;
也可以写成:
select * from city join person on city.id = person.id;
结果如下:
从结果可以看出,表格中显示出了city.id=person.id的记录,它显示出了符合这个条件的记录。
full outer join(全外连接)返回参与连接的两个数据集合中的全部数据
sql语句:
select * from city full outer join person on city.id = person.id;
也可以写成:
select * from city full join person on city.id = person.id;
结果如下:
从结果可以看出,全外连接得到了city和person表中的全部数据
left outer join(左连接) 返回包括左表中的所有记录和右表中连接字段相等的记录
sql语句:
select * from city left outer join person on city.id = person.id;
也可以写成:
select * from city left join person on city.id = person.id;
结果如下:
从结果可以看出,左外连接和全外连接的结果一模一样?
我们在给person中添加一行数据:
insert into person values(9,'Kiki');
在重新执行:
select * from city full join person on city.id = person.id;
结果如下:
select * from city left join person on city.id = person.id;
结果如下:
两个结果对照着看,left join显示出了city中的所有记录和person连接字段相等的记录
right outer join(右连接) 返回包括右表中的所有记录和左表中连接字段相等的记录
sql语句:
select * from city right outer join person on city.id = person.id;
也可以写成
select * from city right join person on city.id = person.id;
结果如下:
从结果可以看出,person中的记录被全部显示出来,而city中的显示的数据是根据连接字段相等的记录
补充:PostgreSQL表连接:内连接,外连接,自连接,交叉连接
搜了搜,基本上都是写内连接、外连接、交叉连接这三种类型,但我发现PostgreSQL还有自连接。不妨一并写来做个记录。
内连接,就是两个表逐行匹配,匹配上的内容都显示,没有匹配的都不显示。
外连接有三种,左外连接,右外连接,全外连接。
左外连接是以左表为基础,左表内容全部显示,右表有匹配到左表的则显示,否则不显示。
右外连接是以右表为基础,右表内容全部显示,左表有匹配到右表的则显示,否则不显示。
全外连接是以两表为基础,显示三部分内容,一部分是内连接的内容,即两表匹配的内容,一部分是左表有而右表无的,一部分是左表无右表有的。
自连接是逐行,用当前这行数据和这个表中其他行进行匹配。
交叉连接最省事,笛卡尔积,左表m行右表n行,则结果是m*n行。
下面是两个表的内容。
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 行记录) mydb=# select * from cities; name | location ---------------+----------- San Francisco | (-194,53) London | (0,51) (2 行记录)
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 行记录) 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 行记录)
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 行记录) 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 行记录) 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 行记录)
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 行记录)
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 行记录)
以上为个人经验,希望能给大家一个参考,也希望大家多多支持。如有错误或未考虑完全的地方,望不吝赐教。