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union operator

Applies to: ✅ Microsoft Fabric ✅ Azure Data Explorer ✅ Azure Monitor ✅ Microsoft Sentinel

Takes two or more tables and returns the rows of all of them.

Syntax

[ T | ] union [ UnionParameters ] [kind= inner|outer] [withsource= ColumnName] [isfuzzy= true|false] Tables

Learn more about syntax conventions.

Note

The operation of the union operator can be altered by setting the best_effort request property to true, using either a set statement or through client request properties. When this property is set to true, the union operator will disregard fuzzy resolution and connectivity failures to execute any of the sub-expressions being “unioned” and yield a warning in the query status results.

Parameters

Name	Type	Required	Description
T	string		The input tabular expression.
UnionParameters	string		Zero or more space-separated parameters in the form of Name = Value that control the behavior of the row-match operation and execution plan. See supported union parameters.
kind	string		Either inner or outer. inner causes the result to have the subset of columns that are common to all of the input tables. outer causes the result to have all the columns that occur in any of the inputs. Cells that aren't defined by an input row are set to null. The default is outer. With outer, the result has all the columns that occur in any of the inputs, one column for each name and type occurrences. This means that if a column appears in multiple tables and has multiple types, it has a corresponding column for each type in the union's result. This column name is suffixed with a '_' followed by the origin column type.
withsource=ColumnName	string		If specified, the output includes a column called ColumnName whose value indicates which source table has contributed each row. If the query effectively references tables from more than one database including the default database, then the value of this column has a table name qualified with the database. cluster and database qualifications are present in the value if more than one cluster is referenced.
isfuzzy	bool		If set to true, allows fuzzy resolution of union legs. The set of union sources is reduced to the set of table references that exist and are accessible at the time while analyzing the query and preparing for execution. If at least one such table was found, any resolution failure yields a warning in the query status results, but won't prevent the query execution. If no resolutions were successful, the query returns an error. The default is false. isfuzzy=true only applies to the union sources resolution phase. Once the set of source tables is determined, possible additional query failures won't be suppressed.
Tables	string		One or more comma-separated table references, a query expression enclosed with parenthesis, or a set of tables specified with a wildcard. For example, E* would form the union of all the tables in the database whose names begin E.

Supported union parameters

Name	Type	Required	Description
hint.concurrency	int		Hints the system how many concurrent subqueries of the union operator should be executed in parallel. The default is the number of CPU cores on the single node of the cluster (2 to 16).
hint.spread	int		Hints the system how many nodes should be used by the concurrent union subqueries execution. The default is 1.

Note

• The union scope can include let statements if attributed with the view keyword.
• The union scope will not include functions. To include a function, define a let statement with the view keyword.
• There's no guarantee of the order in which the union legs will appear, but if each leg has an order by operator, then each leg will be sorted.

Returns

A table with as many rows as there are in all the input tables.

Examples

Tables with string in name or column

union K* | where * has "Kusto"

Rows from all tables in the database whose name starts with K, and in which any column includes the word Kusto.

Distinct count

union withsource=SourceTable kind=outer Query, Command
| where Timestamp > ago(1d)
| summarize dcount(UserId)

The number of distinct users that have produced either a Query event or a Command event over the past day. In the result, the 'SourceTable' column will indicate either "Query" or "Command".

Query
| where Timestamp > ago(1d)
| union withsource=SourceTable kind=outer 
   (Command | where Timestamp > ago(1d))
| summarize dcount(UserId)

This more efficient version produces the same result. It filters each table before creating the union.

Using isfuzzy=true

// Using union isfuzzy=true to access non-existing view:                                     
let View_1 = view () { print x=1 };
let View_2 = view () { print x=1 };
let OtherView_1 = view () { print x=1 };
union isfuzzy=true
(View_1 | where x > 0), 
(View_2 | where x > 0),
(View_3 | where x > 0)
| count 

Output

Count
2

Observing Query Status - the following warning returned: Failed to resolve entity 'View_3'

// Using union isfuzzy=true and wildcard access:
let View_1 = view () { print x=1 };
let View_2 = view () { print x=1 };
let OtherView_1 = view () { print x=1 };
union isfuzzy=true View*, SomeView*, OtherView*
| count 

Output

Count
3

Observing Query Status - the following warning returned: Failed to resolve entity 'SomeView*'

Source columns types mismatch

let View_1 = view () { print x=1 };
let View_2 = view () { print x=toint(2) };
union withsource=TableName View_1, View_2

Output

TableName	x_long	x_int
View_1	1
View_2		2

let View_1 = view () { print x=1 };
let View_2 = view () { print x=toint(2) };
let View_3 = view () { print x_long=3 };
union withsource=TableName View_1, View_2, View_3 

Output

TableName	x_long1	x_int	x_long
View_1	1
View_2		2
View_3			3

Column x from View_1 received the suffix _long, and as a column named x_long already exists in the result schema, the column names were de-duplicated, producing a new column- x_long1