{"id":12405,"date":"2020-02-23T13:42:00","date_gmt":"2020-02-23T21:42:00","guid":{"rendered":"https:\/\/careerkarma.com\/blog\/?p=12405"},"modified":"2023-12-01T02:29:56","modified_gmt":"2023-12-01T10:29:56","slug":"sql-union","status":"publish","type":"post","link":"https:\/\/careerkarma.com\/blog\/sql-union\/","title":{"rendered":"SQL Union"},"content":{"rendered":"\n

When you\u2019re querying a database, you may want to combine the result of two or more SELECT<\/code> statements. For example, you may want to get a list of the cities in which your customers are based, and the cities in which your business has branches. You could run two queries to achieve this goal, but you may want to have a result in one query.<\/p>\n\n\n\n

That\u2019s where the SQL UNION<\/code> operator comes in. The UNION<\/code> clause can be used to combine the results of two or more SELECT<\/code> queries into a single result set.<\/p>\n\n\n\n

In this tutorial, we are going to break down the basics of the SQL UNION<\/code> operator and discuss where you may want to use this command when you\u2019re working with a database.<\/p>\n\n\n\n

Query Refresher<\/h2>\n\n\n\n

Programmers use queries to retrieve information from a database. Queries almost always begin with the SQL SELECT<\/code> statement and are used to retrieve data based on a set of criteria. Queries usually include the FROM<\/code> operator, which states which table will be queried, among other operators that can filter the resulting data.<\/p>\n\n\n\n

Here is the general syntax for an SQL<\/code> query: <\/p>\n\n\n\n

SELECT column_name FROM table_name WHERE your_condititions_are_met;<\/pre><\/div>\n\n\n\n
Here\u2019s an example of an SQL<\/code> query that returns a list of all employee names: <\/p>\n\n\n\n
SELECT name FROM employees;<\/pre><\/div>\n\n\n\n
The output from our query is as follows:<\/p>\n\n\n\n
name<\/td><\/tr>
Luke
Mike
Hannah
Geoff
Alexis
Emma
Jonah<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
(7 rows)<\/p>\n\n\n\n
If you want to retrieve information from multiple columns, you can so do by separating the column names with a comma. If you want to get data from every column, you can use an asterisk (*<\/code>) instead, which represents every column in a table.<\/p>\n\n\n\n
SQL Union<\/h2>\n\n\n\n
The SQL UNION<\/code> operator can be used to combine the results of two or more queries into a single response that results in one table.<\/p>\n\n\n\n
In order to use the UNION<\/code> operator, two conditions must be met. Firstly, the data types of the new columns should be compatible\u2014if a salary is an integer in one table and a float in the other, the union would not work. Secondly, the number and order of the columns must be the same in your queries.<\/p>\n\n\n\n
Here\u2019s the syntax for an SQL UNION<\/code> query:<\/p>\n\n\n\n
SELECT column_name FROM table1_name\nUNION SELECT column_name FROM table2_name;<\/pre><\/div>\n\n\n\n
Let\u2019s use an example to illustrate how the SQL UNION<\/code> operator works. Let\u2019s say that we are a business that needs to send an announcement to all of our customers. We also want all employees to be sent this announcement so they are aware of what is going on.<\/p>\n\n\n\n
We could use the following SQL<\/code> query to get a list of the emails of both our customers and our employees so we can send them all the announcement:<\/p>\n\n\n\n
SELECT name, email FROM employees\nUNION SELECT name, email FROM customers;<\/pre><\/div>\n\n\n\n
Here is the output from our query:<\/p>\n\n\n\n
name <\/td>email<\/td><\/tr>
Emma<\/td>emma.a@gmail.com<\/td><\/tr>
Jonah<\/td>jonah.h@gmail.com<\/td><\/tr>
Hannah<\/td>hannah.y@gmail.com<\/td><\/tr>
Luke<\/td>luke.e@gmail.com<\/td><\/tr>
John<\/td>john.p@outlook.com<\/td><\/tr>
Geoff<\/td>geoff.l@gmail.com<\/td><\/tr>
Alexis<\/td>alexis.e@gmail.com<\/td><\/tr>
Fred<\/td>fred.s@gmail.com<\/td><\/tr>
Erin<\/td>erin.a@gmail.com<\/td><\/tr>
Katy<\/td>katy.l@gmail.com<\/td><\/tr>
Anne<\/td>anne.s@gmail.com<\/td><\/tr>
Tom<\/td>tom.h@gmail.com<\/td><\/tr>
Mike<\/td>mike.h@gmail.com<\/td><\/tr>
Hannah<\/td>hannah.p@gmail.com<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
(14 rows)<\/p>\n\n\n\n
As you can see, our UNION<\/code> query returned a list of all the names and email addresses for both our customers and our employees.<\/p>\n\n\n\n
It\u2019s worth noting that the UNION<\/code> operator removes duplicate rows from the combined final result. This means that if one of our employees was also a customer, we would only see their information once.<\/p>\n\n\n\n
While this worked out in the above example, if you want to return a result including duplicate rows, you would need to add the ALL<\/code> keyword to your query. Here\u2019s an example:<\/p>\n\n\n\n
SELECT name, email FROM employees\nUNION ALL\nSELECT name, email FROM customers;<\/pre><\/div>\n\n\n\n
Conclusion<\/h2>\n\n\n\n
In this tutorial we\u2019ve broken down how to use the UNION<\/code> operator on an SQL<\/code> server. As we\u2018ve discussed, UNION<\/code> can be used to get data from two tables and combine the response into a single table. For example, if you wanted to get a list of your suppliers\u2019 and distributors\u2019 addresses\u2014which were both stored in separate tables\u2014you could use a UNION<\/code> query.<\/p>\n","protected":false},"excerpt":{"rendered":"When you\u2019re querying a database, you may want to combine the result of two or more SELECT statements. For example, you may want to get a list of the cities in which your customers are based, and the cities in which your business has branches. You could run two queries to achieve this goal, but…","protected":false},"author":240,"featured_media":12406,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[17284],"tags":[],"class_list":{"0":"post-12405","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-sql"},"acf":{"post_sub_title":"","sprint_id":"","query_class":"SQL","school_sft":"","parent_sft":"","school_privacy_policy":"","has_review":"","is_sponser_post":"","is_guest_post":""},"yoast_head":"\nSQL Union: combine the result of two or more SELECT statements<\/title>\n<meta name=\"description\" content=\"The union SQL function allows coders to combine the results of two or more queries into one final table. 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