SELECT

The main functionality of Beam SQL is the SELECT statement. This is how you query and join data. The operations supported are a subset of Apache Calcite SQL.

Generally, the semantics of queries is standard. Please see the following sections to learn about extensions for supporting Beam’s unified batch/streaming model:

Query statements scan one or more tables or expressions and return the computed result rows. This topic describes the syntax for SQL queries in Beam.

SQL Syntax

query_statement:
    [ WITH with_query_name AS ( query_expr ) [, ...] ]
    query_expr

query_expr:
    { select | ( query_expr ) | query_expr set_op query_expr }
    [ LIMIT count [ OFFSET skip_rows ] ]

select:
    SELECT  [{ ALL | DISTINCT }]
        { [ expression. ]* [ EXCEPT ( column_name [, ...] ) ]
            [ REPLACE ( expression [ AS ] column_name [, ...] ) ]
        | expression [ [ AS ] alias ] } [, ...]
    [ FROM from_item  [, ...] ]
    [ WHERE bool_expression ]
    [ GROUP BY { expression [, ...] | ROLLUP ( expression [, ...] ) } ]
    [ HAVING bool_expression ]

set_op:
    UNION { ALL | DISTINCT } | INTERSECT DISTINCT | EXCEPT DISTINCT

from_item: {
    table_name [ [ AS ] alias ] |
    join |
    ( query_expr ) [ [ AS ] alias ]
    with_query_name [ [ AS ] alias ]
}

join:
    from_item [ join_type ] JOIN from_item
    [ { ON bool_expression | USING ( join_column [, ...] ) } ]

join_type:
    { INNER | CROSS | FULL [OUTER] | LEFT [OUTER] | RIGHT [OUTER] }

Notation:

SELECT list

Syntax:

SELECT  [{ ALL | DISTINCT }]
    { [ expression. ]*
    | expression [ [ AS ] alias ] } [, ...]

The SELECT list defines the columns that the query will return. Expressions in the SELECT list can refer to columns in any of the from_items in its corresponding FROM clause.

Each item in the SELECT list is one of:

SELECT *

SELECT *, often referred to as select star, produces one output column for each column that is visible after executing the full query.

SELECT * FROM (SELECT 'apple' AS fruit, 'carrot' AS vegetable);

+-------+-----------+
| fruit | vegetable |
+-------+-----------+
| apple | carrot    |
+-------+-----------+

SELECT expression

Items in a SELECT list can be expressions. These expressions evaluate to a single value and produce one output column, with an optional explicit alias.

If the expression does not have an explicit alias, it receives an implicit alias according to the rules for implicit aliases, if possible. Otherwise, the column is anonymous and you cannot refer to it by name elsewhere in the query.

SELECT expression.*

An item in a SELECT list can also take the form of expression.*. This produces one output column for each column or top-level field of expression. The expression must be a table alias.

The following query produces one output column for each column in the table groceries, aliased as g.

WITH groceries AS
  (SELECT 'milk' AS dairy,
   'eggs' AS protein,
   'bread' AS grain)
SELECT g.*
FROM groceries AS g;

+-------+---------+-------+
| dairy | protein | grain |
+-------+---------+-------+
| milk  | eggs    | bread |
+-------+---------+-------+

SELECT modifiers

You can modify the results returned from a SELECT query, as follows.

SELECT DISTINCT

A SELECT DISTINCT statement discards duplicate rows and returns only the remaining rows. SELECT DISTINCT cannot return columns of the following types:

SELECT ALL

A SELECT ALL statement returns all rows, including duplicate rows. SELECT ALL is the default behavior of SELECT.

Aliases

See Aliases for information on syntax and visibility for SELECT list aliases.

FROM clause

The FROM clause indicates the table or tables from which to retrieve rows, and specifies how to join those rows together to produce a single stream of rows for processing in the rest of the query.

Syntax

from_item: {
    table_name [ [ AS ] alias ] |
    join |
    ( query_expr ) [ [ AS ] alias ] |
    with_query_name [ [ AS ] alias ]
}

table_name

The name (optionally qualified) of an existing table.

SELECT * FROM Roster;
SELECT * FROM beam.Roster;

join

See JOIN Types below and Joins.

select

( select ) [ [ AS ] alias ] is a table subquery.

with_query_name

The query names in a WITH clause (see WITH Clause) act like names of temporary tables that you can reference anywhere in the FROM clause. In the example below, subQ1 and subQ2 are with_query_names.

Example:

WITH
  subQ1 AS (SELECT * FROM Roster WHERE SchoolID = 52),
  subQ2 AS (SELECT SchoolID FROM subQ1)
SELECT DISTINCT * FROM subQ2;

The WITH clause hides any permanent tables with the same name for the duration of the query, unless you qualify the table name, e.g. beam.Roster.

Subqueries

A subquery is a query that appears inside another statement, and is written inside parentheses. These are also referred to as “sub-SELECTs” or “nested SELECTs”. The full SELECT syntax is valid in subqueries.

There are two types of subquery:

Note that there must be parentheses around both types of subqueries.

Example:

SELECT AVG ( PointsScored )
FROM
( SELECT PointsScored
  FROM Stats
  WHERE SchoolID = 77 )

Optionally, a table subquery can have an alias.

Example:

SELECT r.LastName
FROM
( SELECT * FROM Roster) AS r;

Aliases

See Aliases for information on syntax and visibility for FROM clause aliases.

JOIN types

Also see Joins.

Syntax

join:
    from_item [ join_type ] JOIN from_item
    [ ON bool_expression | USING ( join_column [, ...] ) ]

join_type:
    { INNER | CROSS | FULL [OUTER] | LEFT [OUTER] | RIGHT [OUTER] }

The JOIN clause merges two from_items so that the SELECT clause can query them as one source. The join_type and ON or USING clause (a “join condition”) specify how to combine and discard rows from the two from_items to form a single source.

All JOIN clauses require a join_type.

A JOIN clause requires a join condition unless one of the following conditions is true:

[INNER] JOIN

An INNER JOIN, or simply JOIN, effectively calculates the Cartesian product of the two from_items and discards all rows that do not meet the join condition. “Effectively” means that it is possible to implement an INNER JOIN without actually calculating the Cartesian product.

CROSS JOIN

CROSS JOIN is generally not yet supported.

FULL [OUTER] JOIN

A FULL OUTER JOIN (or simply FULL JOIN) returns all fields for all rows in both from_items that meet the join condition.

FULL indicates that all rows from both from_items are returned, even if they do not meet the join condition. For streaming jobs, all rows that are not late according to default trigger and belonging to the same window if there’s non-global window applied.

OUTER indicates that if a given row from one from_item does not join to any row in the other from_item, the row will return with NULLs for all columns from the other from_item.

Also see Joins.

LEFT [OUTER] JOIN

The result of a LEFT OUTER JOIN (or simply LEFT JOIN) for two from_items always retains all rows of the left from_item in the JOIN clause, even if no rows in the right from_item satisfy the join predicate.

LEFT indicates that all rows from the left from_item are returned; if a given row from the left from_item does not join to any row in the right from_item, the row will return with NULLs for all columns from the right from_item. Rows from the right from_item that do not join to any row in the left from_item are discarded.

RIGHT [OUTER] JOIN

The result of a RIGHT OUTER JOIN (or simply RIGHT JOIN) is similar and symmetric to that of LEFT OUTER JOIN.

ON clause

The ON clause contains a bool_expression. A combined row (the result of joining two rows) meets the join condition if bool_expression returns TRUE.

Example:

SELECT * FROM Roster INNER JOIN PlayerStats
ON Roster.LastName = PlayerStats.LastName;

USING clause

The USING clause requires a column_list of one or more columns which occur in both input tables. It performs an equality comparison on that column, and the rows meet the join condition if the equality comparison returns TRUE.

In most cases, a statement with the USING keyword is equivalent to using the ON keyword. For example, the statement:

SELECT FirstName
FROM Roster INNER JOIN PlayerStats
USING (LastName);

is equivalent to:

SELECT FirstName
FROM Roster INNER JOIN PlayerStats
ON Roster.LastName = PlayerStats.LastName;

The results from queries with USING do differ from queries that use ON when you use SELECT *. To illustrate this, consider the query:

SELECT * FROM Roster INNER JOIN PlayerStats
USING (LastName);

This statement returns the rows from Roster and PlayerStats where Roster.LastName is the same as PlayerStats.LastName. The results include a single LastName column.

By contrast, consider the following query:

SELECT * FROM Roster INNER JOIN PlayerStats
ON Roster.LastName = PlayerStats.LastName;

This statement returns the rows from Roster and PlayerStats where Roster.LastName is the same as PlayerStats.LastName. The results include two LastName columns; one from Roster and one from PlayerStats.

Sequences of JOINs

The FROM clause can contain multiple JOIN clauses in sequence.

Example:

SELECT * FROM a LEFT JOIN b ON TRUE LEFT JOIN c ON TRUE;

where a, b, and c are any from_items. JOINs are bound from left to right, but you can insert parentheses to group them in a different order.

WHERE clause

Syntax

WHERE bool_expression

The WHERE clause filters out rows by evaluating each row against bool_expression, and discards all rows that do not return TRUE (that is, rows that return FALSE or NULL).

Example:

SELECT * FROM Roster
WHERE SchoolID = 52;

The bool_expression can contain multiple sub-conditions.

Example:

SELECT * FROM Roster
WHERE LastName LIKE 'Mc%' OR LastName LIKE 'Mac%';

You cannot reference column aliases from the SELECT list in the WHERE clause.

Expressions in an INNER JOIN have an equivalent expression in the WHERE clause. For example, a query using INNER JOIN and ON has an equivalent expression using CROSS JOIN and WHERE.

Example - this query:

SELECT * FROM Roster INNER JOIN TeamMascot
ON Roster.SchoolID = TeamMascot.SchoolID;

is equivalent to:

SELECT * FROM Roster CROSS JOIN TeamMascot
WHERE Roster.SchoolID = TeamMascot.SchoolID;

GROUP BY clause

Also see Windowing & Triggering

Syntax

GROUP BY { expression [, ...] | ROLLUP ( expression [, ...] ) }

The GROUP BY clause groups together rows in a table with non-distinct values for the expression in the GROUP BY clause. For multiple rows in the source table with non-distinct values for expression, the GROUP BY clause produces a single combined row. GROUP BY is commonly used when aggregate functions are present in the SELECT list, or to eliminate redundancy in the output.

Example:

SELECT SUM(PointsScored), LastName
FROM PlayerStats
GROUP BY LastName;

HAVING clause

Syntax

HAVING bool_expression

The HAVING clause is similar to the WHERE clause: it filters out rows that do not return TRUE when they are evaluated against the bool_expression.

As with the WHERE clause, the bool_expression can be any expression that returns a boolean, and can contain multiple sub-conditions.

The HAVING clause differs from the WHERE clause in that:

The HAVING clause can reference columns available via the FROM clause, as well as SELECT list aliases. Expressions referenced in the HAVING clause must either appear in the GROUP BY clause or they must be the result of an aggregate function:

SELECT LastName
FROM Roster
GROUP BY LastName
HAVING SUM(PointsScored) > 15;

Set operators

Syntax

UNION { ALL | DISTINCT } | INTERSECT DISTINCT | EXCEPT DISTINCT

Set operators combine results from two or more input queries into a single result set. You must specify ALL or DISTINCT; if you specify ALL, then all rows are retained. If DISTINCT is specified, duplicate rows are discarded.

If a given row R appears exactly m times in the first input query and n times in the second input query (m >= 0, n >= 0):

The following rules apply:

Examples:

query1 UNION ALL (query2 UNION DISTINCT query3)
query1 UNION ALL query2 UNION ALL query3

Invalid:

query1 UNION ALL query2 UNION DISTINCT query3
query1 UNION ALL query2 INTERSECT ALL query3;  // INVALID.

UNION

The UNION operator combines the result sets of two or more input queries by pairing columns from the result set of each query and vertically concatenating them.

INTERSECT

The INTERSECT operator returns rows that are found in the result sets of both the left and right input queries. Unlike EXCEPT, the positioning of the input queries (to the left vs. right of the INTERSECT operator) does not matter.

EXCEPT

The EXCEPT operator returns rows from the left input query that are not present in the right input query.

LIMIT clause and OFFSET clause

Syntax

LIMIT count [ OFFSET skip_rows ]

LIMIT specifies a non-negative count of type INTEGER, and no more than count rows will be returned. LIMIT 0 returns 0 rows. If there is a set operation, LIMIT is applied after the set operation is evaluated.

OFFSET specifies a non-negative skip_rows of type INTEGER, and only rows from that offset in the table will be considered.

These clauses accept only literal or parameter values.

The rows that are returned by LIMIT and OFFSET is unspecified.

WITH clause

The WITH clause contains one or more named subqueries which execute every time a subsequent SELECT statement references them. Any clause or subquery can reference subqueries you define in the WITH clause. This includes any SELECT statements on either side of a set operator, such as UNION.

Example:

WITH subQ1 AS (SELECT SchoolID FROM Roster),
     subQ2 AS (SELECT OpponentID FROM PlayerStats)
SELECT * FROM subQ1
UNION ALL
SELECT * FROM subQ2;

Aliases

An alias is a temporary name given to a table, column, or expression present in a query. You can introduce explicit aliases in the SELECT list or FROM clause, or Beam will infer an implicit alias for some expressions. Expressions with neither an explicit nor implicit alias are anonymous and the query cannot reference them by name.

Explicit alias syntax

You can introduce explicit aliases in either the FROM clause or the SELECT list.

In a FROM clause, you can introduce explicit aliases for any item, including tables, arrays, subqueries, and UNNEST clauses, using [AS] alias. The AS keyword is optional.

Example:

SELECT s.FirstName, s2.SongName
FROM Singers AS s JOIN Songs AS s2 ON s.SingerID = s2.SingerID;

You can introduce explicit aliases for any expression in the SELECT list using [AS] alias. The AS keyword is optional.

Example:

SELECT s.FirstName AS name, LOWER(s.FirstName) AS lname
FROM Singers s;

Explicit alias visibility

After you introduce an explicit alias in a query, there are restrictions on where else in the query you can reference that alias. These restrictions on alias visibility are the result of Beam’s name scoping rules.

FROM clause aliases

Beam processes aliases in a FROM clause from left to right, and aliases are visible only to subsequent JOIN clauses.

Ambiguous aliases

Beam provides an error if a name is ambiguous, meaning it can resolve to more than one unique object.

Examples:

This query contains column names that conflict between tables, since both Singers and Songs have a column named SingerID:

SELECT SingerID
FROM Singers, Songs;

Implicit aliases

In the SELECT list, if there is an expression that does not have an explicit alias, Beam assigns an implicit alias according to the following rules. There can be multiple columns with the same alias in the SELECT list.

In all other cases, there is no implicit alias, so the column is anonymous and cannot be referenced by name. The data from that column will still be returned and the displayed query results may have a generated label for that column, but the label cannot be used like an alias.

In a FROM clause, from_items are not required to have an alias. The following rules apply:

If there is an expression that does not have an explicit alias, Beam assigns an implicit alias in these cases:

Table subqueries do not have implicit aliases.

FROM UNNEST(x) does not have an implicit alias.

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