[sql] What is the difference between UNION and UNION ALL?

What is the difference between UNION and UNION ALL?

If there is no ORDER BY, a UNION ALL may bring rows back as it goes, whereas a UNION would make you wait until the very end of the query before giving you the whole result set at once. This can make a difference in a time-out situation - a UNION ALL keeps the connection alive, as it were.

So if you have a time-out issue, and there's no sorting, and duplicates aren't an issue, UNION ALL may be rather helpful.

I add an example,

UNION, it is merging with distinct --> slower, because it need comparing (In Oracle SQL developer, choose query, press F10 to see cost analysis).

UNION ALL, it is merging without distinct --> faster.

SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual
UNION
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual;

and

SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual
UNION ALL
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual;

Important! Difference between Oracle and Mysql: Let's say that t1 t2 don't have duplicate rows between them but they have duplicate rows individual. Example: t1 has sales from 2017 and t2 from 2018

SELECT T1.YEAR, T1.PRODUCT FROM T1

UNION ALL

SELECT T2.YEAR, T2.PRODUCT FROM T2

In ORACLE UNION ALL fetches all rows from both tables. The same will occur in MySQL.

However:

SELECT T1.YEAR, T1.PRODUCT FROM T1

UNION

SELECT T2.YEAR, T2.PRODUCT FROM T2

In ORACLE, UNION fetches all rows from both tables because there are no duplicate values between t1 and t2. On the other hand in MySQL the resultset will have fewer rows because there will be duplicate rows within table t1 and also within table t2!

(From Microsoft SQL Server Book Online)

UNION [ALL]

Specifies that multiple result sets are to be combined and returned as a single result set.

ALL

Incorporates all rows into the results. This includes duplicates. If not specified, duplicate rows are removed.

UNION will take too long as a duplicate rows finding like DISTINCT is applied on the results.

SELECT * FROM Table1
UNION
SELECT * FROM Table2

is equivalent of:

SELECT DISTINCT * FROM (
    SELECT * FROM Table1
    UNION ALL
    SELECT * FROM Table2) DT

A side effect of applying DISTINCT over results is a sorting operation on results.

UNION ALL results will be shown as arbitrary order on results But UNION results will be shown as ORDER BY 1, 2, 3, ..., n (n = column number of Tables) applied on results. You can see this side effect when you don't have any duplicate row.

Both UNION and UNION ALL concatenate the result of two different SQLs. They differ in the way they handle duplicates.

• UNION performs a DISTINCT on the result set, eliminating any duplicate rows.

• UNION ALL does not remove duplicates, and it therefore faster than UNION.

Note: While using this commands all selected columns need to be of the same data type.

Example: If we have two tables, 1) Employee and 2) Customer

1. Employee table data:

2. Customer table data:

3. UNION Example (It removes all duplicate records):

4. UNION ALL Example (It just concatenate records, not eliminate duplicates, so it is faster than UNION):

You can avoid duplicates and still run much faster than UNION DISTINCT (which is actually same as UNION) by running query like this:

SELECT * FROM mytable WHERE a=X UNION ALL SELECT * FROM mytable WHERE b=Y AND a!=X

Notice the AND a!=X part. This is much faster then UNION.

It is good to understand with a Venn diagramm.

here is the link to the source. There is a good description.

Just to add my two cents to the discussion here: one could understand the UNION operator as a pure, SET-oriented UNION - e.g. set A={2,4,6,8}, set B={1,2,3,4}, A UNION B = {1,2,3,4,6,8}

When dealing with sets, you would not want numbers 2 and 4 appearing twice, as an element either is or is not in a set.

In the world of SQL, though, you might want to see all the elements from the two sets together in one "bag" {2,4,6,8,1,2,3,4}. And for this purpose T-SQL offers the operator UNION ALL.

UNION removes duplicate records in other hand UNION ALL does not. But one need to check the bulk of data that is going to be processed and the column and data type must be same.

since union internally uses "distinct" behavior to select the rows hence it is more costly in terms of time and performance. like

select project_id from t_project
union
select project_id from t_project_contact  

this gives me 2020 records

on other hand

select project_id from t_project
union all
select project_id from t_project_contact

gives me more than 17402 rows

on precedence perspective both has same precedence.

UNION merges the contents of two structurally-compatible tables into a single combined table.

• Difference:

The difference between UNION and UNION ALL is that UNION will omit duplicate records whereas UNION ALL will include duplicate records.

Union Result set is sorted in ascending order whereas UNION ALL Result set is not sorted

UNION performs a DISTINCT on its Result set so it will eliminate any duplicate rows. Whereas UNION ALL won't remove duplicates and therefore it is faster than UNION.*

Note: The performance of UNION ALL will typically be better than UNION, since UNION requires the server to do the additional work of removing any duplicates. So, in cases where it is certain that there will not be any duplicates, or where having duplicates is not a problem, use of UNION ALL would be recommended for performance reasons.

One more thing i would like to add-

Union:- Result set is sorted in ascending order.

Union All:- Result set is not sorted. two Query output just gets appended.

Not sure that it matters which database

UNION and UNION ALL should work on all SQL Servers.

You should avoid of unnecessary UNIONs they are huge performance leak. As a rule of thumb use UNION ALL if you are not sure which to use.

UNION ALL also works on more data types as well. For example when trying to union spatial data types. For example:

select a.SHAPE from tableA a
union
select b.SHAPE from tableB b

will throw

The data type geometry cannot be used as an operand to the UNION, INTERSECT or EXCEPT operators because it is not comparable.

However union all will not.

UNION removes duplicates, whereas UNION ALL does not.

In order to remove duplicates the result set must be sorted, and this may have an impact on the performance of the UNION, depending on the volume of data being sorted, and the settings of various RDBMS parameters ( For Oracle PGA_AGGREGATE_TARGET with WORKAREA_SIZE_POLICY=AUTO or SORT_AREA_SIZE and SOR_AREA_RETAINED_SIZE if WORKAREA_SIZE_POLICY=MANUAL ).

Basically, the sort is faster if it can be carried out in memory, but the same caveat about the volume of data applies.

Of course, if you need data returned without duplicates then you must use UNION, depending on the source of your data.

I would have commented on the first post to qualify the "is much less performant" comment, but have insufficient reputation (points) to do so.

union is used to select distinct values from two tables where as union all is used to select all values including duplicates from the tables

UNION - results in distinct records

while

UNION ALL - results in all the records including duplicates.

Both are blocking operators and hence I personally prefer using JOINS over Blocking Operators(UNION, INTERSECT, UNION ALL etc. ) anytime.

To illustrate why Union operation performs poorly in comparison to Union All checkout the following example.

CREATE TABLE #T1 (data VARCHAR(10))

INSERT INTO #T1
SELECT 'abc'
UNION ALL
SELECT 'bcd'
UNION ALL
SELECT 'cde'
UNION ALL
SELECT 'def'
UNION ALL
SELECT 'efg'


CREATE TABLE #T2 (data VARCHAR(10))

INSERT INTO #T2
SELECT 'abc'
UNION ALL
SELECT 'cde'
UNION ALL
SELECT 'efg'

Following are results of UNION ALL and UNION operations.

A UNION statement effectively does a SELECT DISTINCT on the results set. If you know that all the records returned are unique from your union, use UNION ALL instead, it gives faster results.

Using UNION results in Distinct Sort operations in the Execution Plan. Proof to prove this statement is shown below:

In ORACLE: UNION does not support BLOB (or CLOB) column types, UNION ALL does.

The basic difference between UNION and UNION ALL is union operation eliminates the duplicated rows from the result set but union all returns all rows after joining.

from http://zengin.wordpress.com/2007/07/31/union-vs-union-all/

UNION and UNION ALL used to combine two or more query results.

UNION command selects distinct and related information from two tables which will eliminates duplicate rows.

On the other hand, UNION ALL command selects all the values from both the tables, which displays all rows.

Suppose that you have two table Teacher & Student

Both have 4 Column with different Name like this

Teacher - ID(int), Name(varchar(50)), Address(varchar(50)), PositionID(varchar(50))

Student- ID(int), Name(varchar(50)), Email(varchar(50)), PositionID(int)

You can apply UNION or UNION ALL for those two table which have same number of columns. But they have different name or data type.

When you apply UNION operation on 2 tables, it neglects all duplicate entries(all columns value of row in a table is same of another table). Like this

SELECT * FROM Student
UNION
SELECT * FROM Teacher

the result will be

When you apply UNION ALL operation on 2 tables, it returns all entries with duplicate(if there is any difference between any column value of a row in 2 tables). Like this

SELECT * FROM Student
UNION ALL
SELECT * FROM Teacher

Output

Performance:

Obviously UNION ALL performance is better that UNION as they do additional task to remove the duplicate values. You can check that from Execution Estimated Time by press ctrl+L at MSSQL

UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.

UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.

The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table.

A UNION statement effectively does a SELECT DISTINCT on the results set. If you know that all the records returned are unique from your union, use UNION ALL instead, it gives faster results.