LEFT JOIN

The LEFT JOIN keyword returns all records from the left table (table1), and the matching records from the right table (table2).

Description

The LEFT JOIN keyword returns all records from the left table (table1), and the matching records from the right table (table2). The result is 0 records from the right side, if there is no match.

note

In some databases LEFT JOIN is called LEFT OUTER JOIN.

Syntax

> SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name;

Examples

drop table if exists t1;
drop table if exists t2;
drop table if exists t3;
create table t1 (libname1 varchar(21) not null primary key, city varchar(20));
create table t2 (isbn2 varchar(21) not null primary key, author varchar(20), title varchar(60));
create table t3 (isbn3 varchar(21) not null, libname3 varchar(21) not null, quantity int);
insert into t2 values ('001','Daffy','Aducklife');
insert into t2 values ('002','Bugs','Arabbitlife');
insert into t2 values ('003','Cowboy','Lifeontherange');
insert into t2 values ('000','Anonymous','Wannabuythisbook?');
insert into t2 values ('004','BestSeller','OneHeckuvabook');
insert into t2 values ('005','EveryoneBuys','Thisverybook');
insert into t2 values ('006','SanFran','Itisasanfranlifestyle');
insert into t2 values ('007','BerkAuthor','Cool.Berkley.the.book');
insert into t3 values ('000','NewYorkPublicLibra',1);
insert into t3 values ('001','NewYorkPublicLibra',2);
insert into t3 values ('002','NewYorkPublicLibra',3);
insert into t3 values ('003','NewYorkPublicLibra',4);
insert into t3 values ('004','NewYorkPublicLibra',5);
insert into t3 values ('005','NewYorkPublicLibra',6);
insert into t3 values ('006','SanFransiscoPublic',5);
insert into t3 values ('007','BerkeleyPublic1',3);
insert into t3 values ('007','BerkeleyPublic2',3);
insert into t3 values ('001','NYC Lib',8);
insert into t1 values ('NewYorkPublicLibra','NewYork');
insert into t1 values ('SanFransiscoPublic','SanFran');
insert into t1 values ('BerkeleyPublic1','Berkeley');
insert into t1 values ('BerkeleyPublic2','Berkeley');
insert into t1 values ('NYCLib','NewYork');

mysql> select city,libname1,count(libname1) as a from t3 left join t1 on libname1=libname3 join t2 on isbn3=isbn2 group by city,libname1;
+----------+--------------------+------+
| city     | libname1           | a    |
+----------+--------------------+------+
| NewYork  | NewYorkPublicLibra |    6 |
| SanFran  | SanFransiscoPublic |    1 |
| Berkeley | BerkeleyPublic1    |    1 |
| Berkeley | BerkeleyPublic2    |    1 |
| NULL     | NULL               |    0 |
+----------+--------------------+------+