SQL SERVER – Introduction to JOINs – Basic of JOINs

INNER JOIN

This join returns rows when there is at least one match in both the tables.

OUTER JOIN

There are three different Outer Join methods.

LEFT OUTER JOIN
This join returns all the rows from the left table in conjunction with the matching rows from the right table. If there are no columns matching in the right table, it returns NULL values.

RIGHT OUTER JOIN
This join returns all the rows from the right table in conjunction with the matching rows from the left table. If there are no columns matching in the left table, it returns NULL values.

FULL OUTER JOIN
This join combines left outer join and right outer join. It returns row from either table when the conditions are met and returns null value when there is no match.

CROSS JOIN

This join is a Cartesian join that does not necessitate any condition to join. The resultset contains records that are multiplication of record number from both the tables.

Additional Notes related to JOIN:

The following are three classic examples to display where Outer Join is useful. You will notice several instances where developers write query as given below.

SELECT t1.*FROM Table1 t1WHERE t1.ID NOT IN (SELECT t2.ID FROM Table2 t2)GO

The query demonstrated above can be easily replaced by Outer Join. Indeed, replacing it by Outer Join is the best practice. The query that gives same result as above is displayed here using Outer Join and WHERE clause in join.

/* LEFT JOIN - WHERE NULL */SELECT t1.*,t2.*FROM Table1 t1LEFT JOIN Table2 t2 ON t1.ID = t2.IDWHERE t2.ID IS NULL

The above example can also be created using Right Outer Join.

NOT INNER JOIN
Remember, the term Not Inner Join does not exist in database terminology. However, when full Outer Join is used along with WHERE condition, as explained in the above two examples, it will give you exclusive result to Inner Join. This join will give all the results that were not present in Inner Join.

You can download the complete SQL Script here, but for the sake of complicity I am including the same script here.

USE AdventureWorks
GOCREATE TABLE table1(ID INT, Value VARCHAR(10))INSERT INTO Table1 (ID, Value)SELECT 1,'First'UNION ALLSELECT 2,'Second'UNION ALLSELECT 3,'Third'UNION ALLSELECT 4,'Fourth'UNION ALLSELECT 5,'Fifth'GOCREATE TABLE table2(ID INT, Value VARCHAR(10))INSERT INTO Table2 (ID, Value)SELECT 1,'First'UNION ALLSELECT 2,'Second'UNION ALLSELECT 3,'Third'UNION ALLSELECT 6,'Sixth'UNION ALLSELECT 7,'Seventh'UNION ALLSELECT 8,'Eighth'GOSELECT *FROM Table1SELECT *FROM Table2
GOUSE AdventureWorks
GO/* INNER JOIN */SELECT t1.*,t2.*FROM Table1 t1INNER JOIN Table2 t2 ON t1.ID = t2.ID
GO/* LEFT JOIN */SELECT t1.*,t2.*FROM Table1 t1LEFT JOIN Table2 t2 ON t1.ID = t2.ID
GO/* RIGHT JOIN */SELECT t1.*,t2.*FROM Table1 t1RIGHT JOIN Table2 t2 ON t1.ID = t2.ID
GO/* OUTER JOIN */SELECT t1.*,t2.*FROM Table1 t1FULL OUTER JOIN Table2 t2 ON t1.ID = t2.ID
GO/* LEFT JOIN - WHERE NULL */SELECT t1.*,t2.*FROM Table1 t1LEFT JOIN Table2 t2 ON t1.ID = t2.IDWHERE t2.ID IS NULLGO/* RIGHT JOIN - WHERE NULL */SELECT t1.*,t2.*FROM Table1 t1RIGHT JOIN Table2 t2 ON t1.ID = t2.IDWHERE t1.ID IS NULLGO/* OUTER JOIN - WHERE NULL */SELECT t1.*,t2.*FROM Table1 t1FULL OUTER JOIN Table2 t2 ON t1.ID = t2.IDWHERE t1.ID IS NULL OR t2.ID IS NULLGO/* CROSS JOIN */SELECT t1.*,t2.*FROM Table1 t1CROSS JOIN Table2 t2
GODROP TABLE table1DROP TABLE table2
GO