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UNION vs. UNION ALL: Know the Difference

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By Shumaila Saeed || Published on February 7, 2024
UNION combines results from multiple SELECT statements, removing duplicates, while UNION ALL combines results including all duplicates.
UNION vs. UNION ALL

Key Differences

UNION merges different SELECT statement results, eliminating duplicate rows. UNION ALL also merges results but includes all duplicates.
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UNION performs additional work to remove duplicates, which can be slower. UNION ALL is faster as it doesn't check for duplicates.
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UNION is used when distinct results are needed. UNION ALL is preferred when all results, including duplicates, are required.
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UNION results in a set of unique rows. UNION ALL can produce a larger result set due to duplicates.
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Both use similar syntax, differing only in the keyword ALL for UNION ALL.
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Comparison Chart

Duplication Handling

Removes duplicates
Includes duplicates
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Performance

Slower due to duplicate removal
Faster as no duplicate check is performed
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Typical Use Cases

When unique results are necessary
When all results, including duplicates, are needed
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Result Set Size

Potentially smaller set
Potentially larger set
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Query Purpose

For distinct, non-repeated data
For complete data including repetitions
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UNION and UNION ALL Definitions

UNION

Merges results from multiple SELECT queries, removing duplicates.
SELECT name FROM table1 UNION SELECT name FROM table2.
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UNION ALL

Ideal for complete data set without filtering duplicates.
SELECT country FROM users UNION ALL SELECT country FROM admins.
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UNION

Suitable for queries where duplicate rows are not needed.
SELECT country FROM users UNION SELECT country FROM admins.
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UNION ALL

Faster as it doesn't remove duplicate rows.
SELECT city FROM customers UNION ALL SELECT city FROM suppliers.
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UNION

The act of uniting or the state of being united.
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UNION ALL

Used for comprehensive data retrieval including repetitions.
SELECT product FROM inventory UNION ALL SELECT product FROM orders.
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UNION

A combination so formed, especially an alliance or confederation of people, parties, or political entities for mutual interest or benefit.
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UNION ALL

Combines results from multiple queries, keeping all duplicates.
SELECT name FROM table1 UNION ALL SELECT name FROM table2.
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UNION

(Mathematics) A set, every member of which is an element of one or another of two or more given sets.
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UNION ALL

Also requires matching columns but keeps all results.
SELECT id, name FROM employees UNION ALL SELECT id, name FROM managers.
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UNION

Agreement or harmony resulting from the uniting of individuals; concord.
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UNION

The state of matrimony; marriage
"The element that was to make possible such a union was trust in each other's love" (Kate Chopin).
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UNION

Sexual intercourse.
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UNION

A combination of parishes for joint administration of relief for the poor in Great Britain.
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UNION

A workhouse maintained by such a union.
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UNION

A labor union.
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UNION

A coupling device for connecting parts, such as pipes or rods.
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UNION

A device on a flag or ensign, occupying the upper inner corner or the entire field, that signifies the union of two or more sovereignties.
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UNION

An organization at a college or university that provides facilities for recreation; a student union.
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UNION

A building housing such facilities.
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UNION

Union The United States of America regarded as a national unit, especially during the Civil War.
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UNION

Union Of, relating to, or loyal to the United States of America during the Civil War
A Union soldier.
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UNION

Of or relating to a labor union or labor union organizing
The union movement.
Union negotiations.
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UNION

(countable) The act of uniting or joining two or more things into one.
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UNION

(countable) The state of being united or joined; a state of unity or harmony.
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UNION

(countable) That which is united, or made one; something formed by a combination or coalition of parts or members; a confederation; a consolidated body; a league.
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UNION

(countable) A trade union; a workers' union.
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UNION

(countable) An association of students at a university for social and/or political purposes; also in some cases a debating body.
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UNION

(countable) A joint or other connection uniting parts of machinery, such as pipes.
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UNION

The set containing all of the elements of two or more sets.
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UNION

(countable) The act or state of marriage.
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UNION

Sexual intercourse.
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UNION

A data structure that can store any of various types of item, but only one at a time.
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UNION

A large, high-quality pearl.
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UNION

(historical) An affiliation of several parishes for joint support and management of their poor; also the jointly-owned workhouse.
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UNION

To combine sets using the union operation.
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UNION

Belonging to, represented by, or otherwise pertaining to a labour union.
Actors have to be union to get work here.
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UNION

The act of uniting or joining two or more things into one, or the state of being united or joined; junction; coalition; combination.
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UNION

Agreement and conjunction of mind, spirit, will, affections, or the like; harmony; concord.
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UNION

That which is united, or made one; something formed by a combination or coalition of parts or members; a confederation; a consolidated body; a league; as, the weavers have formed a union; trades unions have become very numerous; the United States of America are often called the Union.
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UNION

A textile fabric composed of two or more materials, as cotton, silk, wool, etc., woven together.
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UNION

A large, fine pearl.
If they [pearls] be white, great, round, smooth, and weighty . . . our dainties and delicates here at Rome . . . call them unions, as a man would say "singular," and by themselves alone.
In the cup an union shall he throw,Richer than that which four successive kingsIn Denmark's crown have worn.
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UNION

A device emblematic of union, used on a national flag or ensign, sometimes, as in the military standard of Great Britain, covering the whole field; sometimes, as in the flag of the United States, and the English naval and marine flag, occupying the upper inner corner, the rest of the flag being called the fly. Also, a flag having such a device; especially, the flag of Great Britain.
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UNION

A joint or other connection uniting parts of machinery, or the like, as the elastic pipe of a tender connecting it with the feed pipe of a locomotive engine; especially, a pipe fitting for connecting pipes, or pipes and fittings, in such a way as to facilitate disconnection.
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UNION

A cask suspended on trunnions, in which fermentation is carried on.
One kingdom, joy, and union without end.
[Man] is to . . . begetLike of his like, his image multiplied.In unity defective; which requiresCollateral love, and dearest amity.
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UNION

An organization of employees formed to bargain with the employer;
You have to join the union in order to get a job
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UNION

The United States (especially the northern states during the American Civil War);
He has visited every state in the Union
Lee hoped to detach Maryland from the Union
The North's superior resources turned the scale
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UNION

The act of pairing a male and female for reproductive purposes;
The casual couplings of adolescents
The mating of some species occurs only in the spring
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UNION

The state of being joined or united or linked;
There is strength in union
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UNION

The state of being a married couple voluntarily joined for life (or until divorce);
A long and happy marriage
God bless this union
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UNION

Healing process involving the growing together of the edges of a wound or the growing together of broken bones
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UNION

A political unit formed from previously independent people or organizations;
The Soviet Union
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UNION

A set containing all and only the members of two or more given sets;
Let C be the union of the sets A and B
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UNION

The occurrence of a uniting of separate parts;
Lightning produced an unusual union of the metals
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UNION

A device on a national flag emblematic of the union of two or more sovereignties (typically in the upper inner corner)
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UNION

The act of making or becoming a single unit;
The union of opposing factions
He looked forward to the unification of his family for the holidays
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UNION

Being of or having to do with the northern United States and those loyal to the Union during the Civil War;
Union soldiers
Federal forces
A Federal infantryman
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UNION

Of trade unions;
The union movement
Union negotiations
A union-shop clause in the contract
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UNION

Ensures distinct results from combined queries.
SELECT city FROM customers UNION SELECT city FROM suppliers.
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UNION

Used for data aggregation from different tables.
SELECT product FROM inventory UNION SELECT product FROM orders.
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UNION

Requires same number and type of columns in all SELECT statements.
SELECT id, name FROM employees UNION SELECT id, name FROM managers.
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Repeatedly Asked Queries

How does UNION ALL differ from UNION?

UNION ALL includes all duplicates, while UNION removes them.
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What does UNION do in SQL?

Combines results from multiple SELECT statements, removing duplicates.
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Can UNION and UNION ALL have different numbers of columns?

No, both require the same number and type of columns in all SELECT statements.
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Is UNION ALL suitable for distinct data?

No, it includes all duplicates, so it's not for distinct data.
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Can UNION queries come from different tables?

Yes, UNION can combine results from different tables.
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When should I use UNION?

When you need distinct results without duplicates from combined queries.
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Do UNION and UNION ALL support ordering of results?

Yes, both can use ORDER BY to sort the final result.
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Is UNION faster than UNION ALL?

No, UNION ALL is generally faster as it doesn't remove duplicates.
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Can UNION result in fewer rows than the original queries?

Yes, due to the removal of duplicates.
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Can UNION be used with WHERE clause?

Yes, each SELECT in the UNION can have its own WHERE clause.
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Can aggregate functions be used with UNION?

Yes, aggregate functions can be used in the SELECT statements within UNION.
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Can I limit the number of rows returned in UNION/UNION ALL?

Yes, by using LIMIT in the overall or individual SELECT statements.
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Are column names important in UNION operations?

Only the order and data types are important, not the column names.
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Does UNION ALL maintain the order of rows?

It combines rows as they are, but an ORDER BY is needed for specific ordering.
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Is it possible to combine more than two queries with UNION/UNION ALL?

Yes, you can combine multiple SELECT statements using UNION or UNION ALL.
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Are the data types important in UNION operations?

Yes, corresponding columns must have compatible data types.
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Can I use JOIN with UNION or UNION ALL?

Yes, JOINs can be used within the SELECT statements of a UNION or UNION ALL.
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How do I choose between UNION and UNION ALL?

Choose based on whether you need to eliminate duplicates (UNION) or include all data (UNION ALL).
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Is UNION ALL good for performance optimization?

Yes, especially when duplicate data is not a concern.
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How are NULL values treated in UNION?

NULL values are considered duplicates if they appear in the same column.
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About Author
Shumaila Saeed
Written by
Shumaila Saeed
Shumaila Saeed, an expert content creator with 6 years of experience, specializes in distilling complex topics into easily digestible comparisons, shining a light on the nuances that both inform and educate readers with clarity and accuracy.

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