Many times in a relational database the information you want to show in your query is in more than one table.  This begs the question “How do you combine results from more than one table?”

All the examples for this lesson are based on Microsoft SQL Server Management Studio and the AdventureWorks2012 database.  You can get started using these free tools using my Guide Getting Started Using SQL Server.

What are the ways I can combine results from more than one query?

SQL wouldn’t be a very useful language if it didn’t provide an easy means for you to combine results from more than one query.  Fortunately, there are three main ways you can combine data from multiple tables.  We’ll go over these briefly here and provide links to more in-depth articles.

Three Main Ways to Combine Data

Data in relational database tables are organized into rows and columns.  As we investigate ways to combine data, keep in mind that the end result will be to either add more columns to a result, perhaps from another related table, or rows, by taking a set of rows from two or more tables.

When most people learn to combine data they learn about:

  1. JOIN – You can use joins to combine columns from one or more queries into one result.
  2. UNION – Use Unions and other set operators to combine rows from one or more queries into one result.
  3. Sub Queries – Sometimes called nested queries, these can be used to perform a separate search in the database showed results can be used in another query.


I like to think of joins as the glue that puts the database back together.  Relational databases are usually normalized to make the data easier to maintain and to improve performance, but the end result is information is separated into many tables.  You can use Joins to recombine that information back together into a more human-readable format.  The data is recombined by matching columns from each table.

In all cases, joins require two main ingredients:  Two tables and a join condition.  The tables are what we will use to pull the rows and columns and the join condition is how we intend on matching the columns between tables.

Example JOIN

SELECT  Person.FirstName,
 FROM   Person.Person
        INNER JOIN Person.PersonPhone
        ON Person.BusinessEntityID = 

There are two main types of joins.  Inner Joins and Outer Joins.

Inner Joins only return a resulting row if the join condition matches in both tables.  Inner joins are mainly used to match the primary key of one table a foreign key in another.

The second type of join is an outer join.  Outer joins always return at least one row for the main table, referred to as the Left or Right table, and null values in the corresponding columns of the non-matching column.  Outer joins are useful for finding non-matching data.

It is important to note that joins can return more rows than exist in either table combined.  The joins return combinations of matches.  If you join two tables, one containing 5 row, and the other 10, the result may contain anywhere from 0 to 50 rows depending on the join condition.


A UNION is used to combine the rows of two or more queries into one result.  The Union is called a set operator.

There are some special conditions that must occur in order for a union to work.  First, each query must have the same number of columns.  Second, the data types of these columns must be compatible.  Generally speaking, each query must return the same number and type of columns.

A practical example of union is when two tables contain part numbers and you want to create a combined list for a catalog.  You can either elect to have the end result be a unique listing for the combined query or if you use UNION ALL return all rows from each table.

Example UNION

FROM   Production.ProductCategory AS C
FROM   Production.ProductSubcategory AS S

In addition to Union there are a couple of other handy set operators:

  • INTERSECT – You can use this to only return rows that are common between two tables.
  • EXCEPT – You can use this to return rows that exist on one table, but aren’t found in another.

As you go on to learn more SQL you find that you can use joins to write equivalent statements for Intersect and Except, but there are no equivalents for Union.


Subqueries are sometimes called nested queries.  They are queries defined inside of other queries.  Subqueries can be confusing.  I think a lot of this stems from the fact they can be used in many places in a SQL select statement, and for several purposes!

For example, here are some areas you may see a subquery:

  • SELECT clause – Used to return a value.  For instance, if you’re querying a sales table, you could include the total sales by return a sum of all sales from within a subquery.
  • WHERE clause – Subqueries can be used in the where clause in comparisons.  You could set up a comparison to compare sales to the overall average.  The overall average would be returned from a subquery.  You can also use subqueries in membership operators such as IN.  Rather than hard-coding the in clause you can use a subquery to make it more dynamic.
  • HAVING clause – A single value from a subquery is included in the HAVING clause comparisons.

Example Subquery

SELECT SalesOrderID,
       (SELECT AVG(LineTotal)
        FROM Sales.SalesOrderDetail) AS AverageLineTotal
FROM   Sales.SalesOrderDetail

When used in select clauses and comparison operators such as equals, greater than, and less than, a subquery can only return one row.  If used in conjunction with a membership operator, such as IN, it is OK for the query to return one or more rows.

About the author 

Kris Wenzel

Kris Wenzel has been working with databases over the past 30 years as a developer, analyst, and DBA. He has a BSE in Computer Engineering from the University of Michigan and a MBA from the University of Notre Dame. Kris has written hundreds of blog articles and many online courses. He loves helping others learn SQL.

  • I have 2 tables. first table contains name and age and the second table contains class and roll num. I have a 3rd table which contains all these 4 columns. Now I want to fill the 3rd table with the 1st and 2nd table’s data.

    Please give the suggestion

    • From your question you want to insert some rows into a table based on data from more than one table. To do this you can use the INSERT statement in combination with SELECT INNER JOIN query.
      Here are the tables I’m using in the example.





      The first step is to create a SELECT clause to test the join. Let make sure we get all the data combined the way we want before we move on to combining it with the INSERT. Here is the select statement I created:

      SELECT P.Name, P.Age, P.Address, C.Class, C.RollNum, C.Location
      FROM Person P
      INNER JOIN Enrollment E ON P.Name = E.Name and P.Age = E.Age
      INNER JOIN Class C ON C.Class = E.Class and C.RollNum = E.RollNum

      Now after you have tested it and made sure it is returning all the rows you wish, we can change it into the INSERT statement. We’ll take the results of this SELECT and insert them in the table FinalTable.

      INSERT INTO FinalTable (Name, Age, Address, Class, RollNum, Location)
      SELECT P.Name, P.Age, P.Address, C.Class, C.RollNum, C.Location
      FROM Person P
      INNER JOIN Enrollment E ON P.Name = E.Name and P.Age = E.Age
      INNER JOIN Class C ON C.Class = E.Class and C.RollNum = E.RollNum

  • Hello, i have a question. I have table users that have userid, fname, lname, email, phone and table units that have unit number, userid, currentOwner.

    i would like to publish the result in this view:

    user name | all unit numbers for the userid | phone | email

    any help please is appreciated

  • i want to combine two result sets where two result sets are from same table just with different conditions but need to combine them in a way that
    first query gives me customer,account1(account column alias as account1) – based on some condition
    second query gives me customer,account2(account column alias as account2) – based on different condition
    now on joining them i need output like
    customer Account1 Account2
    1 A1
    2 A2

    something like that, could someone help me

    • I think for this example you can use UNION ALL. Here is an overall structure of the query you can use:
      SELECT customer, Account as Account1, NULL as Account2
      FROM accounts
      WHERE AccountType = 1
      SELECT customer, NULL, Account
      FROM accounts
      WHERE AccountType = 2

  • I have ages calculated in my table. all I need is 2 different sets of age groups from the age field. please can someone suggest how can I do this?

    • CASE WHEN [Age] >=1 AND [Age] =11 AND [Age] =21 AND [Age] <=30 THEN '21-30 Years'
      END AS 'Age Group'

      Or something like that.

      • Hi,

        You can try something similiar to:

        SELECT name, CASE age
        WHEN age <= 12 THEN 'youth' WHEN age >12 AND age < 18 THEN 'teen ager' WHEN age >= 18 and < 25 THEN 'young adult' WHEN age >= 25 ‘adult’
        Else ‘unknown’
        End as [Age Group]
        From people

        Here is an article you can also look at on SQL Case Statements.

    • Use a View… write the query to produce the results as you want them then create it as a View. Then use the View.

      For reporting purposes you don’t need or want extra tables, you want a View.

  • Hi,

    i have three tables which have Serial no in common. i have Firstname in first table, secondname in second table and thirdname in third do i join three tables so that the result is a single column of name from all three tables

    Output should be:



    select @cols = STUFF((SELECT ‘,’ + QUOTENAME(designation)
    from MyTable
    group by designation
    order by designation
    ).value(‘.’, ‘NVARCHAR(MAX)’),1,1,”)

    set @query = N’SELECT Row, ‘ + @cols + N’ from
    select ”SS” Row, SS AS Value , designation from MyTable
    select ”AS” Row, [AS] AS Value , designation from MyTable
    select ”Vac” Row, Vac AS Value , designation from MyTable
    ) x
    max(Value) for designation in (‘ + @cols + N’)
    ) p ‘
    exec sp_executesql @query;

    For more details:

  • Is it okay if I want to combine data from three unique ID from a table (Ex: P01, P02, P03) and join with another table with a unique ID. Means the P01, P02, P03 become under one ID in another table.

  • hi please help as i have how to find % increase in average.

    I have avg from table 1 and avg from table 2 by using union in select command but i need to see % increase of avg also in same query please tell how to do it.

  • {"email":"Email address invalid","url":"Website address invalid","required":"Required field missing"}


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