SELECT

Retrieve rows from a table.

Table of contents

• Synopsis

• Description

• Parameters

  • The SELECT List

  • Clauses

    • OVER

    • FROM

      • Relation reference

      • Joined relation

      • Table function

      • Subselect

    • WHERE

    • GROUP BY

      • HAVING

    • UNION ALL

    • ORDER BY

    • WINDOW

    • LIMIT

    • OFFSET

Synopsis

SELECT [ ALL | DISTINCT ] * | expression [ [ AS ] output_name ] [, ...]
  [ OVER ( window_definition ) [, ...] ]
  [ FROM relation ]
  [ WHERE condition ]
  [ GROUP BY expression [, ...] [HAVING condition] ]
  [ UNION ALL query_specification ]
  [ WINDOW window_name AS ( window_definition ) [, ...] ]
  [ ORDER BY expression [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] ]
  [ LIMIT num_results ]
  [ OFFSET start ]

where relation is:

relation_reference | joined_relation | table_function | subselect

Description

SELECT retrieves rows from a table. The general processing of SELECT is as follows:

• The FROM item points to the table where the data should be retrieved from. If no FROM item is specified, the query is executed against a virtual table with no columns.

• If the WHERE clause is specified, all rows that do not satisfy the condition are eliminated from the output.

• If the GROUP BY clause is specified, the output is combined into groups of rows that match on one or more values.

• The actual output rows are computed using the SELECT output expressions for each selected row or row group.

• If the ORDER BY clause is specified, the returned rows are sorted in the specified order. If ORDER BY is not given, the rows are returned in whatever order the system finds fastest to produce.

• If DISTINCT is specified, one unique row is kept. All other duplicate rows are removed from the result set.

• If the LIMIT or OFFSET clause is specified, the SELECT statement only returns a subset of the result rows.

Parameters

The SELECT List

The SELECT list specifies expressions that form the output rows of the SELECT statement. The expressions can (and usually do) refer to columns computed in the FROM clause.

SELECT [ ALL | DISTINCT ] * | expression [ [ AS ] output_name ] [, ...]

Just as in a table, every output column of a SELECT has a name. In a simple SELECT, this name is just used to label the column for display. To specify the name to use for an output column, write AS output_name after the column’s expression. (You can omit AS, but only if the desired output name does not match any reserved keyword. For protection against possible future keyword additions, it is recommended that you always either write AS or double-quote the output name.) If you do not specify a column name, a name is chosen automatically by CrateDB. If the column’s expression is a simple column reference, then the chosen name is the same as that column’s name. In more complex cases, a function or type name may be used, or the system may fall back on a generated name.

An output column’s name can be used to refer to the column’s value in ORDER BY and GROUP BY clauses, but not in the WHERE clause; there you must write out the expression instead.

Instead of an expression, * can be written in the output list as a shorthand for all the columns of the selected rows. Also, you can write table_name.* as a shorthand for the columns coming from just that table. In these cases it is not possible to specify new names with AS; the output column names will be the same as the table columns’ names.

Clauses

OVER

The OVER clause defines a window.

OVER ( window_definition )

The window_definition determines the partitioning and ordering of rows before the window function is applied.

See also

Window functions: Window definition

FROM

The FROM clause specifies the source relation for the SELECT:

FROM relation

The relation can be any of the following relations.

Relation reference

A relation_reference is an ident which can either reference a table or a view with an optional alias:

relation_ident [ [AS] alias ]

relation_ident

The name (optionally schema-qualified) of an existing table or view.

alias

A substitute name for the FROM item containing the alias.

An alias is used for brevity. When an alias is provided, it completely hides the actual name of the relation. For example given FROM foo AS f, the remainder of the SELECT must refer to this FROM item as f not foo.

See also

SQL syntax: CREATE TABLE

SQL syntax: CREATE VIEW

Joined relation

A joined_relation is a relation which joins two relations together.

relation { , | join_type JOIN } relation [ { ON join_condition  |  USING (col_names) } ]

join_type

LEFT [OUTER], RIGHT [OUTER], FULL [OUTER], CROSS or INNER.

join_condition

An expression which specifies which rows in a join are considered a match.

The join_condition is not applicable for joins of type CROSS and must have a returning value of type boolean.

col_names

A comma-separated list of column names. The joined relations need to contain the specified columns.

Table function

table_function is a function that produces a set of rows and has columns.

function_call

function_call

The call declaration of the function. Usually in the form of function_name ( [ args ] ).

Depending on the function the parenthesis and arguments are either optional or required.

See also

Built-ins: Table functions

Subselect

A subselect is another SELECT statement surrounded by parentheses with an alias:

( select_stmt ) [ AS ] alias

The subselect behaves like a temporary table that is evaluated at runtime. The clauses of the surrounding SELECT statements are applied on the result of the inner SELECT statement.

select_stmt

A SELECT statement.

alias

An alias for the subselect.

WHERE

The optional WHERE clause defines the condition to be met for a row to be returned:

WHERE condition

condition

A WHERE condition is any expression that evaluates to a result of type boolean.

Any row that does not satisfy this condition will be eliminated from the output. A row satisfies the condition if it returns true when the actual row values are substituted for any variable references.

GROUP BY

The optional GROUP BY clause will condense all selected rows that share the same values for the grouped expression into a single row.

Aggregate expressions, if any are used, are computed across all rows making up each group, producing a separate value for each group.

GROUP BY expression [, ...] [HAVING condition]

expression

An arbitrary expression formed from column references of the queried relation that are also present in the result column list. Numeric literals are interpreted as ordinals referencing an output column from the select list.

It can also reference output columns by name.

In case of ambiguity, a GROUP BY name will be interpreted as a name of a column from the queried relation rather than an output column name.

HAVING

The optional HAVING clause defines the condition to be met for values within a resulting row of a GROUP BY clause.

condition

A HAVING condition is any expression that evaluates to a result of type boolean. Every row for which the condition is not satisfied will be eliminated from the output.

Note

When GROUP BY is present, it is not valid for the SELECT list expressions to refer to ungrouped columns except within aggregate functions, since there would otherwise be more than one possible value to return for an ungrouped column.

Additionally, grouping can only be applied on indexed fields.

See also

Fulltext indices : Disable indexing

UNION ALL

The UNION ALL operator combines the result sets of two or more SELECT statements. The two SELECT statements that represent the direct operands of the UNION ALL must produce the same number of columns, and corresponding columns must be of the same data types. The result of UNION ALL may contain duplicate rows. You can find here sample usage of UNION ALL.

UNION ALL query_specification

query_specification

Can be any SELECT statement.

ORDER BY, LIMIT, and OFFSET can only be applied after the last SELECT statement of the UNION ALL, as they are applied to the complete result of the UNION operation. In order to apply an ORDER BY and/or LIMIT and/or OFFSET to any of the partial SELECT statements, those statements need to become subqueries.

Column names used in ORDER BY must be position numbers or refer to the outputs of the first SELECT statement, and no functions can be applied on top of the ORDER BY symbols. To achieve more complex ordering, UNION ALL must become a subselect and the more complex ORDER BY should be applied on the outer SELECT wrapping the UNION ALL subselect.

The ordering of the outcome is not guaranteed unless ORDER BY is used.

ORDER BY

The ORDER BY clause causes the result rows to be sorted according to the specified expression(s).

ORDER BY expression [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...]

expression

Can be the name or ordinal number of an output column, or it can be an arbitrary expression formed from input-column values.

The optional keyword ASC (ascending) or DESC (descending) after any expression allows to define the direction in which values are sorted. The default is ascending.

If NULLS FIRST is specified, null values sort before non-null values. If NULLS LAST is specified, null values sort after non-null values. If neither is specified nulls are considered larger than any value. That means the default for ASC is NULLS LAST and the default for DESC is NULLS FIRST.

If two rows are equal according to the leftmost expression, they are compared according to the next expression and so on. If they are equal according to all specified expressions, they are returned in an implementation-dependent order.

Character-string data is sorted by its UTF-8 representation.

Note

Sorting can only be applied on indexed fields.

Additionally, sorting on geo_point, geo_shape, array, and object is not supported.

See also

Fulltext indices : Disable indexing

WINDOW

The optional WINDOW clause has a form:

WINDOW window_name AS ( window_definition ) [, ...]

The window_name is a name that can be referenced from OVER clauses or subsequent window definitions.

The window_definition determines the partitioning and ordering of rows before the window function is applied.

See also

Window functions: Window definition

Window functions: Named windows

LIMIT

The optional LIMIT clause allows to limit the number of returned result rows:

LIMIT num_results

num_results

Specifies the maximum number of result rows to return.

Note

It is possible for repeated executions of the same LIMIT query to return different subsets of the rows of a table, if there is not an ORDER BY to enforce selection of a deterministic subset.

OFFSET

The optional OFFSET clause allows to skip result rows at the beginning:

OFFSET start

start

Specifies the number of rows to skip before starting to return rows.