GoogleSQL for BigQuery supports subqueries.
About subqueries
A subquery is a query that appears inside anotherquery statement. Subqueries are also referred to as sub-SELECTs ornested SELECTs. The full SELECT syntax is valid in subqueries.
Expression subqueries
Expression subqueries are used ina query wherever expressions are valid. They return a single value, as opposedto a column or table. Expression subqueries can becorrelated.
Scalar subqueries
( subquery )
Description
A subquery inside an expression is interpreted as a scalar subquery.Scalar subqueries are often used in the SELECT list or WHERE clause.
A scalar subquery must select a single column. Trying to select multiplecolumns will result in an analysis error. A SELECT list with a singleexpression is the simplest way to select a single column. The result typeof the scalar subquery is the type of that expression.
Another possibility is to use SELECT AS STRUCT to define a subquery thatselects a single STRUCT type value whose fields are defined by one or moreexpressions.
If the subquery returns exactly one row, that single value is thescalar subquery result. If the subquery returns zero rows, the result is NULL.If the subquery returns more than one row, the query fails with a runtime error.
Examples
In this example, a correlated scalar subquery returns the mascots for a list ofplayers, using the Players and Mascotstables:
SELECT username, (SELECT mascot FROM Mascots WHERE Players.team = Mascots.team) AS player_mascotFROM Players;/*---------------------------* | username | player_mascot | +---------------------------+ | gorbie | cardinal | | junelyn | finch | | corba | parrot | *---------------------------*/In this example, an aggregate scalarsubquery calculates avg_level, the average level of a user in thePlayers table.
SELECT username, level, (SELECT AVG(level) FROM Players) AS avg_levelFROM Players;/*---------------------------------------* | username | level | avg_level | +---------------------------------------+ | gorbie | 29 | 24.66 | | junelyn | 2 | 24.66 | | corba | 43 | 24.66 | *---------------------------------------*/Array subqueries
ARRAY ( subquery )
Description
An ARRAY subquery is a special case of expression subquery, in that it returnsan ARRAY. If the subquery returns zerorows, returns an empty ARRAY.Never returns a NULL ARRAY.
The SELECT list in an ARRAY subquery must have exactly one column ofany type, which defines the element type of the array returned by thearray subquery. If not, an error is returned. When the subquery is written withSELECT AS STRUCT,the SELECT list can include multiple columns, and the value returned bythe array subquery is an ARRAY of the constructedSTRUCTs.Selecting multiple columns without using SELECT AS is an error.
ARRAY subqueries can use SELECT AS STRUCT to buildarrays of structs.
See Array functions for full semantics.
Examples
In this example, an ARRAY subquery returns an array of usernames assigned to thered team in the NPCs table:
SELECT ARRAY(SELECT username FROM NPCs WHERE team = 'red') AS red;/*-----------------* | red | +-----------------+ | [niles,jujul] | *-----------------*/IN subqueries
value [ NOT ] IN ( subquery )
Description
Returns TRUE if value is in the set of rows returned by the subquery.Returns FALSE if the subquery returns zero rows.
The subquery's SELECT list must have a single column of any type andits type must be comparable to the type for value. If not, an error isreturned. For full semantics, including NULL handling, see theIN operator.
If you need to use an IN subquery with an array, these are equivalent:
value [ NOT ] IN ( subquery )value [ NOT ] IN UNNEST( ARRAY( subquery ) )Examples
In this example, the IN operator that checks to see if a username calledcorba exists within the Players table:
SELECT 'corba' IN (SELECT username FROM Players) AS result;/*--------* | result | +--------+ | TRUE | *--------*/EXISTS subqueries
EXISTS( subquery )
Description
Returns TRUE if the subquery produces one or more rows. Returns FALSE if thesubquery produces zero rows. Never returns NULL. Unlike all otherexpression subqueries, there are no rules about the column list.Any number of columns may be selected and it will not affect the query result.
Examples
In this example, the EXISTS operator that checks to see if any rows areproduced, using the Players table:
SELECT EXISTS(SELECT username FROM Players WHERE team = 'yellow') AS result;/*--------* | result | +--------+ | FALSE | *--------*/Table subqueries
FROM ( subquery ) [ [ AS ] alias ]
Description
With table subqueries, the outer query treats the result of the subquery as atable. You can only use these in the FROM clause.
Examples
In this example, a subquery returns a table of usernames from thePlayers table:
SELECT results.usernameFROM (SELECT * FROM Players) AS results;/*-----------* | username | +-----------+ | gorbie | | junelyn | | corba | *-----------*/In this example, a list of NPCsassigned to the red team are returned.
SELECT usernameFROM ( WITH red_team AS (SELECT * FROM NPCs WHERE team = 'red') SELECT * FROM red_team);/*-----------* | username | +-----------+ | niles | | jujul | *-----------*/A correlated subquery is a subquery that references a column from outside thatsubquery. Correlation prevents reusing of the subquery result. You can learnmore about this here.
Examples
In this example, a list of mascots that don't have any players assigned to themare returned. The Mascots and Playerstables are referenced.
SELECT mascotFROM MascotsWHERE NOT EXISTS(SELECT username FROM Players WHERE Mascots.team = Players.team);/*----------* | mascot | +----------+ | sparrow | *----------*/In this example, a correlated scalar subquery returns the mascots for a list ofplayers, using the Players and Mascotstables:
SELECT username, (SELECT mascot FROM Mascots WHERE Players.team = Mascots.team) AS player_mascotFROM Players;/*---------------------------* | username | player_mascot | +---------------------------+ | gorbie | cardinal | | junelyn | finch | | corba | parrot | *---------------------------*/Volatile subqueries
A volatile subquery is a subquery that does not always produce the same resultover the same inputs. For example, if a subquery includes a functionthat returns a random number, the subquery is volatile because the resultis not always the same.
Examples
In this example, a random number of usernames are returned from thePlayers table.
SELECT results.usernameFROM (SELECT * FROM Players WHERE RAND() < 0.5) AS results;-- The results are not always the same when you execute-- the preceding query, but will look similar to this:/*----------* | username | +----------+ | gorbie | | junelyn | *----------*/Evaluation rules for subqueries
Some subqueries are evaluated once, others more often.
- A non-correlated, volatile subquery may be re-evaluated once perrow, depending on your query plan.
- A correlated subquery must be logically re-evaluated for every distinct setof parameter values. Depending on your query plan, a correlatedsubquery may be re-evaluated once per row, even if multiplerows have the same parameter values.
Common tables used in examples
Some examples reference a table called Players:
/*-----------------------------* | username | level | team | +-----------------------------+ | gorbie | 29 | red | | junelyn | 2 | blue | | corba | 43 | green | *-----------------------------*/Some examples reference a table called NPCs:
/*-------------------* | username | team | +-------------------+ | niles | red | | jujul | red | | effren | blue | *-------------------*/Some examples reference a table called Mascots:
/*-------------------* | mascot | team | +-------------------+ | cardinal | red | | parrot | green | | finch | blue | | sparrow | yellow | *-------------------*/You can use this WITH clause to emulate temporary table names forPlayers and NPCsin subqueries that support the WITH clause.:
WITH Players AS ( SELECT 'gorbie' AS username, 29 AS level, 'red' AS team UNION ALL SELECT 'junelyn', 2 , 'blue' UNION ALL SELECT 'corba', 43, 'green'), NPCs AS ( SELECT 'niles' AS username, 'red' AS team UNION ALL SELECT 'jujul', 'red' UNION ALL SELECT 'effren', 'blue'), Mascots AS ( SELECT 'cardinal' AS mascot , 'red' AS team UNION ALL SELECT 'parrot', 'green' UNION ALL SELECT 'finch', 'blue' UNION ALL SELECT 'sparrow', 'yellow')SELECT * FROM ( SELECT username, team FROM Players UNION ALL SELECT username, team FROM NPCs);