Left Join in SQL

February 1, 2023

About left join in SQL

In this article, we will learn about left join in SQL.
Left Join in SQL is used to retrieve data from tables that is common top both the tables along with the complete data of the first table i.e. the left table.

 

Left Join in SQL 2

Left Join in SQL

Join enables data merging from tables i.e  used to query data from multiple tables

How Left join works

  • Left Join always Returns all rows from the left table
  • Matching records from the right side table
  • Null values in the place of non-matching rows in another table.
Left Join in SQL
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Functional Dependencies

Joins in DBMS

Relational Algebra

Deadlock in DBMS

Syntax

select col1,col2.... 
from 
tab1 
LEFT JOIN 
tab2 
on tab1.colname=tab2.colname

Left Join Example

Consider two tables emp and dept table : Emp table
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
7839 KING PRESIDENT – 17-NOV-81 5000 – 10
7698 BLAKE MANAGER 7839 01-MAY-81 2850 – 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 – 10
7566 JONES MANAGER 7839 02-APR-81 2975 – 20
7788 SCOTT ANALYST 7566 19-APR-87 3000 – 20
7902 FORD ANALYST 7566 03-DEC-81 3000 – 20
7369 SMITH CLERK 7902 17-DEC-80 800 – 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7876 ADAMS CLERK 7788 23-MAY-87 1100 – 20
7900 JAMES CLERK 7698 03-DEC-81 950 – 30
7934 MILLER CLERK 7782 23-JAN-82 1300 – 10
Dept table
DEPTNO DNAME LOC
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON

Left join SQL query

select rownum,ename,sal,d.deptno,dname,loc
from 
emp e 
left outer join
dept d
on e.deptno=d.deptno;
O/P
ROWNUM ENAME SAL DEDTNO DNAME LOC
1 KING 5000 10 ACCOUNTING NEW YORK
2 CLARK 2450 10 ACCOUNTING NEW YORK
3 MILLER 1300 10 ACCOUNTING NEW YORK
4 JONES 2975 20 RESEARCH DALLAS
5 SCOTT 3000 20 RESEARCH DALLAS
6 FORD 3000 20 RESEARCH DALLAS
7 SMITH 800 20 RESEARCH DALLAS
8 ADAMS 1100 20 RESEARCH DALLAS
9 BLAKE 2850 30 SALES CHICAGO
10 ALLEN 1600 30 SALES CHICAGO
11 WARD 1250 30 SALES CHICAGO
12 MARTIN 1250 30 SALES CHICAGO
13 TURNER 1500 30 SALES CHICAGO
14 JAMES 950 30 SALES CHICAGO

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