SELECT

Retrieve rows from a table.

Table of Contents

• Synopsis
• Description
• Parameters
  • The SELECT List
  • Clauses
    • FROM
      • Relation Reference
      • Joined Relation
      • Table Function
      • Sub Select
    • WHERE
    • GROUP BY
      • HAVING
    • UNION ALL
    • ORDER BY
    • LIMIT
    • OFFSET

Synopsis

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

where relation is:

relation_reference | joined_relation | table_function | sub_select

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. (See WHERE Clause below.)
• 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

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 ]

See also

CREATE TABLE

CREATE VIEW

Joined Relation

A joined_relation is a relation which joins two relations together. See Joins

relation { , | join_type JOIN } relation [ ON join_condition ]

Table Function

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

function_call

Available functions are documented in the table functions section.

Sub Select

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

( select_stmt ) [ AS ] alias

The sub-select 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.

WHERE

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

WHERE condition

GROUP BY

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

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]

HAVING

The optional HAVING clause defines the condition to be met for values whitin a resulting row of a group by clause.

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. For more information, please refer to 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

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.

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 } ] [, ...]

The optional keyword ASC (ascending) or DESC (descending) after any expression allows to define the direction in which values have 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. For more information, please refer to Disable indexing.

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

LIMIT

The optional LIMIT Clause allows to limit the number or retured result rows:

LIMIT num_results

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