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PostgreSQL/Table Joins/Left Join - История изменений
2026-04-04T10:01:42Z
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http://www.sqle.ru/index.php?title=PostgreSQL/Table_Joins/Left_Join&diff=4446&oldid=prev
в 13:45, 26 мая 2010
2010-05-26T13:45:56Z
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http://www.sqle.ru/index.php?title=PostgreSQL/Table_Joins/Left_Join&diff=4447&oldid=prev
Admin: 1 версия
2010-05-26T10:13:43Z
<p>1 версия</p>
<p><b>Новая страница</b></p><div>==Join condition specified with ON can also contain conditions that do not relate directly to the join==<br />
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<!-- start source code --><br />
<br />
<source lang="sql"><br />
<br />
postgres=#<br />
postgres=# CREATE TABLE t1 (<br />
postgres(# num int,<br />
postgres(# name varchar(10)<br />
postgres(# );<br />
CREATE TABLE<br />
postgres=#<br />
postgres=# insert into t1 values(1,"a");<br />
INSERT 0 1<br />
postgres=# insert into t1 values(2,"b");<br />
INSERT 0 1<br />
postgres=# insert into t1 values(3,"c");<br />
INSERT 0 1<br />
postgres=#<br />
postgres=# CREATE TABLE t2 (<br />
postgres(# num int,<br />
postgres(# name varchar(10)<br />
postgres(# );<br />
CREATE TABLE<br />
postgres=#<br />
postgres=# insert into t2 values(1,"xxx");<br />
INSERT 0 1<br />
postgres=# insert into t2 values(3,"yyy");<br />
INSERT 0 1<br />
postgres=# insert into t2 values(5,"zzz");<br />
INSERT 0 1<br />
postgres=#<br />
postgres=# -- The join condition specified with ON can also contain conditions that do not relate directly<br />
postgres=# -- to the join. This can prove useful for some queries but needs to be thought out carefully.<br />
postgres=#<br />
postgres=# SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num AND t2.name = "xxx";<br />
num | name | num | name<br />
-----+------+-----+------<br />
1 | a | 1 | xxx<br />
2 | b | |<br />
3 | c | |<br />
(3 rows)<br />
postgres=#<br />
postgres=# drop table t1;<br />
DROP TABLE<br />
postgres=# drop table t2;<br />
DROP TABLE<br />
postgres=#<br />
postgres=#<br />
<br />
</source><br />
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==LEFT JOIN sample==<br />
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<br />
<!-- start source code --><br />
<br />
<source lang="sql"><br />
<br />
postgres=#<br />
postgres=# CREATE TABLE t1 (<br />
postgres(# num int,<br />
postgres(# name varchar(10)<br />
postgres(# );<br />
CREATE TABLE<br />
postgres=#<br />
postgres=# insert into t1 values(1,"a");<br />
INSERT 0 1<br />
postgres=# insert into t1 values(2,"b");<br />
INSERT 0 1<br />
postgres=# insert into t1 values(3,"c");<br />
INSERT 0 1<br />
postgres=#<br />
postgres=# CREATE TABLE t2 (<br />
postgres(# num int,<br />
postgres(# name varchar(10)<br />
postgres(# );<br />
CREATE TABLE<br />
postgres=#<br />
postgres=# insert into t2 values(1,"xxx");<br />
INSERT 0 1<br />
postgres=# insert into t2 values(3,"yyy");<br />
INSERT 0 1<br />
postgres=# insert into t2 values(5,"zzz");<br />
INSERT 0 1<br />
postgres=#<br />
postgres=# SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num;<br />
num | name | num | name<br />
-----+------+-----+------<br />
1 | a | 1 | xxx<br />
2 | b | |<br />
3 | c | 3 | yyy<br />
(3 rows)<br />
postgres=#<br />
postgres=# drop table t1;<br />
DROP TABLE<br />
postgres=# drop table t2;<br />
DROP TABLE<br />
postgres=#<br />
<br />
</source><br />
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==LEFT JOIN two tables USING a column name==<br />
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<!-- start source code --><br />
<br />
<source lang="sql"><br />
<br />
postgres=#<br />
postgres=# CREATE TABLE t1 (<br />
postgres(# num int,<br />
postgres(# name varchar(10)<br />
postgres(# );<br />
CREATE TABLE<br />
postgres=#<br />
postgres=# insert into t1 values(1,"a");<br />
INSERT 0 1<br />
postgres=# insert into t1 values(2,"b");<br />
INSERT 0 1<br />
postgres=# insert into t1 values(3,"c");<br />
INSERT 0 1<br />
postgres=#<br />
postgres=# CREATE TABLE t2 (<br />
postgres(# num int,<br />
postgres(# name varchar(10)<br />
postgres(# );<br />
CREATE TABLE<br />
postgres=#<br />
postgres=# insert into t2 values(1,"xxx");<br />
INSERT 0 1<br />
postgres=# insert into t2 values(3,"yyy");<br />
INSERT 0 1<br />
postgres=# insert into t2 values(5,"zzz");<br />
INSERT 0 1<br />
postgres=#<br />
postgres=# SELECT * FROM t1 LEFT JOIN t2 USING (num);<br />
num | name | name<br />
-----+------+------<br />
1 | a | xxx<br />
2 | b |<br />
3 | c | yyy<br />
(3 rows)<br />
postgres=#<br />
postgres=# drop table t1;<br />
DROP TABLE<br />
postgres=# drop table t2;<br />
DROP TABLE<br />
postgres=#<br />
<br />
</source></div>
Admin