Difference between JOINS and UNION in SQL

In this post, I have given the difference between the use of Joins and the use of Union keyword in SQL.

I have a table named Table1 with Severity, Response SLA and Resolution SLA as its columns. Now I need to take the number of issues for both Response SLA and Resolution SLA based on Severity.

1. Using Inner join:

SELECT A.[Severity],A.[ResponseSLA],A.[Count],B.[ResolutionSLA],B.[Count] FROM
(
SELECT [Severity],[ResponseSLA], Count(1) As [Count] FROM Table1
WHERE GroupID = 4 AND [Month] = 'Mar - 2010'
GROUP BY [Severity],[ResponseSLA]
) A
INNER JOIN
(
SELECT [Severity],[ResolutionSLA], Count(1) As [Count] FROM Table1
WHERE GroupID = 4 AND [Month] = 'Mar - 2010'
GROUP BY [Severity],[ResolutionSLA]
) B
ON A.[Severity] = B.[Severity]
Order By A.[Severity]

The above query fetches the following result:

Severity	ResponseSLA	Count	ResolutionSLA	Count
Sev2	Not Met SLA	1	Not Met SLA	1
Sev3	Met SLA	29	Met SLA	31
Sev3	Not Met SLA	2	Met SLA	31
Sev4	Met SLA	597	Met SLA	613
Sev4	Not Met SLA	16	Met SLA	613

Note: You can replace "SELECT A.[Severity],A.[ResponseSLA],A.[Count],B.[ResolutionSLA],B.[Count] FROM" With "SELECT * FROM" and fetch all the columns.

2. Using Union:

Consider you need to calculate the total count. Then you should query as below.

SELECT * FROM
(
SELECT [Severity],[ResponseSLA], Count(1) As [Count] FROM Table1
WHERE GroupID = 4 AND [Month] = 'Mar - 2010'
GROUP BY [Severity],[ResponseSLA]
) A
UNION
(
SELECT 'Total' [Severity],'' [ResponseSLA], Count(1) As [Count] FROM Table1
WHERE GroupID = 4 AND [Month] = 'Mar - 2010'
)

The above query fetches the following result:

Severity	ResponseSLA	Count
Sev2	Not Met SLA	1
Sev3	Met SLA	29
Sev3	Not Met SLA	2
Sev4	Met SLA	597
Sev4	Not Met SLA	16
Total		645

Note: In UNION, the number of columns and the column names should match. In this case, the columns of both A & B should match each other.

Hope you understood the difference. Joins always add the columns to the left and the union statements add the columns to the bottom of the result grid.