SELECT

Retrieves rows from a table or view.

Synopsis

SELECT [ALL | DISTINCT [ON (<expression> [, ...])]]
  * | <expression> [[AS] <output_name>] [, ...]
  [FROM <from_item> [, ...]]
  [WHERE <condition>]
  [GROUP BY <grouping_element> [, ...]]
  [HAVING <condition> [, ...]]
  [WINDOW <window_name> AS (<window_specification>)]
  [{UNION | INTERSECT | EXCEPT} [ALL] <select>]
  [ORDER BY <expression> [ASC | DESC | USING <operator>] [, ...]]
  [LIMIT {<count> | ALL}]
  [OFFSET <start>]

where <grouping_element> can be one of:

  ()
  <expression>
  ROLLUP (<expression> [,...])
  CUBE (<expression> [,...])
  GROUPING SETS ((<grouping_element> [, ...]))

where <window_specification> can be:

  [<window_name>]
  [PARTITION BY <expression> [, ...]]
  [ORDER BY <expression> [ASC | DESC | USING <operator>] [, ...]
     [{RANGE | ROWS}
          { UNBOUNDED PRECEDING
          | <expression> PRECEDING
          | CURRENT ROW
          | BETWEEN <window_frame_bound> AND <window_frame_bound> }]]
                    where <window_frame_bound> can be one of:
                        UNBOUNDED PRECEDING
                        <expression> PRECEDING
                        CURRENT ROW
                        <expression> FOLLOWING
                        UNBOUNDED FOLLOWING

where <from_item> can be one of:

[ONLY] <table_name> [[AS] <alias> [( <column_alias> [, ...] )]]
(select) [AS] <alias> [( <column_alias> [, ...] )]
<function_name> ( [<argument> [, ...]] ) [AS] <alias>
             [( <column_alias> [, ...]
                | <column_definition> [, ...] )]
<function_name> ( [<argument> [, ...]] ) AS
              ( <column_definition> [, ...] )
<from_item> [NATURAL] <join_type>
            <from_item>
          [ON <join_condition> | USING ( <join_column> [, ...] )]

Description

SELECT retrieves rows from zero or more tables. The general processing of SELECT is as follows:

1. All elements in the FROM list are computed. (Each element in the FROM list is a real or virtual table.) If more than one element is specified in the FROM list, they are cross-joined together.
2. If the WHERE clause is specified, all rows that do not satisfy the condition are eliminated from the output.
3. If the GROUP BY clause is specified, the output is divided into groups of rows that match on one or more of the defined grouping elements. If the HAVING clause is present, it eliminates groups that do not satisfy the given condition.
4. If a window expression is specified (and optional WINDOW clause), the output is organized according to the positional (row) or value-based (range) window frame.
5. DISTINCT eliminates duplicate rows from the result. DISTINCT ON eliminates rows that match on all the specified expressions. ALL (the default) will return all candidate rows, including duplicates.
6. The actual output rows are computed using the SELECT output expressions for each selected row.
7. Using the operators UNION, INTERSECT, and EXCEPT, the output of more than one SELECT statement can be combined to form a single result set. The UNION operator returns all rows that are in one or both of the result sets. The INTERSECT operator returns all rows that are strictly in both result sets. The EXCEPT operator returns the rows that are in the first result set but not in the second. In all three cases, duplicate rows are eliminated unless ALL is specified.
8. 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.
9. If the LIMIT or OFFSET clause is specified, the SELECT statement only returns a subset of the result rows.

You must have SELECT privilege on a table to read its values.

Parameters

The SELECT List

The SELECT list (between the key words SELECT and FROM) 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.

Using the clause [AS] <output_name>, another name can be specified for an output column. This name is primarily used to label the column for display. It can also be used to refer to the column’s value in ORDER BY and GROUP BY clauses, but not in the WHERE or HAVING clauses; there you must write out the expression instead. The AS keyword is optional in most cases (such as when declaring an alias for column names, constants, function calls, and simple unary operator expressions). In cases where the declared alias is a reserved SQL keyword, the <output_name> must be enclosed in double quotes to avoid ambiguity.

An <expression> in the SELECT list can be a constant value, a column reference, an operator invocation, a function call, an aggregate expression, a window expression, a scalar subquery, and so on. There are a number of constructs that can be classified as an expression but do not follow any general syntax rules.

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.

The FROM Clause

The FROM clause specifies one or more source tables for the SELECT. If multiple sources are specified, the result is the Cartesian product (cross join) of all the sources. But usually qualification conditions are added to restrict the returned rows to a small subset of the Cartesian product. The FROM clause can contain the following elements:

The WHERE Clause

The optional WHERE clause has the general form:

WHERE <condition>

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.

The GROUP BY Clause

The optional GROUP BY clause has the general form:

GROUP BY <grouping_element> [, ...]

where <grouping_element> can be one of:

()
<expression>
ROLLUP (<expression> [,...])
CUBE (<expression> [,...])
GROUPING SETS ((<grouping_element> [, ...]))

GROUP BY will condense into a single row all selected rows that share the same values for the grouped expressions. <expression> can be an input column name, or the name or ordinal number of an output column (SELECT list item), or an arbitrary expression formed from input-column values. In case of ambiguity, a GROUP BY name will be interpreted as an input-column name rather than an output column name.

Aggregate functions, if any are used, are computed across all rows making up each group, producing a separate value for each group (whereas without GROUP BY, an aggregate produces a single value computed across all the selected rows). 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 be more than one possible value to return for an ungrouped column.

HAWQ has the following additional OLAP grouping extensions (often referred to as supergroups):

GROUP BY ROLLUP (a,b,c)

GROUP BY GROUPING SETS( (a,b,c), (a,b), (a), () )

GROUP BY CUBE (a,b,c)

GROUP BY GROUPING SETS( (a,b,c), (a,b), (a,c), (b,c), (a),
(b), (c), () )

GROUP BY GROUPING SETS( (a,c), (a,b) )

The WINDOW Clause

The WINDOW clause is used to define a window that can be used in the OVER() expression of a window function such as rank or avg. For example:

SELECT vendor, rank() OVER (mywindow) FROM sale
GROUP BY vendor
WINDOW mywindow AS (ORDER BY sum(prc*qty));

A WINDOW clause has this general form:

WINDOW <window_name> AS (<window_specification>)

where <window_specification> can be:

[<window_name>]
[PARTITION BY <expression> [, ...]]
[ORDER BY <expression> [ASC | DESC | USING <operator>] [, ...]
    [{RANGE | ROWS}
      { UNBOUNDED PRECEDING
      | <expression> PRECEDING
      | CURRENT ROW
      | BETWEEN <window_frame_bound> AND <window_frame_bound> }]]
             where window_frame_bound can be one of:
               UNBOUNDED PRECEDING
               <expression> PRECEDING
               CURRENT ROW
               <expression> FOLLOWING
               UNBOUNDED FOLLOWING

The HAVING Clause

The optional HAVING clause has the general form:

HAVING <condition>

where <condition> is the same as specified for the WHERE clause. HAVING eliminates group rows that do not satisfy the condition. HAVING is different from WHERE: WHERE filters individual rows before the application of GROUP BY, while HAVING filters group rows created by GROUP BY. Each column referenced in <condition> must unambiguously reference a grouping column, unless the reference appears within an aggregate function.

The presence of HAVING turns a query into a grouped query even if there is no GROUP BY clause. This is the same as what happens when the query contains aggregate functions but no GROUP BY clause. All the selected rows are considered to form a single group, and the SELECT list and HAVING clause can only reference table columns from within aggregate functions. Such a query will emit a single row if the HAVING condition is true, zero rows if it is not true.

The UNION Clause

The UNION clause has this general form:

<select_statement> UNION [ALL] <select_statement>

where <select_statement> is any SELECT statement without an ORDER BY, LIMIT, FOR UPDATE, or FOR SHARE clause. (ORDER BY and LIMIT can be attached to a subquery expression if it is enclosed in parentheses. Without parentheses, these clauses will be taken to apply to the result of the UNION, not to its right-hand input expression.)

The UNION operator computes the set union of the rows returned by the involved SELECT statements. A row is in the set union of two result sets if it appears in at least one of the result sets. The two SELECT statements that represent the direct operands of the UNION must produce the same number of columns, and corresponding columns must be of compatible data types.

The result of UNION does not contain any duplicate rows unless the ALL option is specified. ALL prevents elimination of duplicates. (Therefore, UNION ALL is usually significantly quicker than UNION; use ALL when you can.)

Multiple UNION operators in the same SELECT statement are evaluated left to right, unless otherwise indicated by parentheses.

Currently, FOR UPDATE and FOR SHARE may not be specified either for a UNION result or for any input of a UNION.

The INTERSECT Clause

The INTERSECT clause has this general form:

<select_statement> INTERSECT [ALL] <select_statement>

where <select_statement> is any SELECT statement without an ORDER BY, LIMIT, FOR UPDATE, or FOR SHARE clause.

The INTERSECT operator computes the set intersection of the rows returned by the involved SELECT statements. A row is in the intersection of two result sets if it appears in both result sets.

The result of INTERSECT does not contain any duplicate rows unless the ALL option is specified. With ALL, a row that has m duplicates in the left table and n duplicates in the right table will appear min(m, n) times in the result set.

Multiple INTERSECT operators in the same SELECT statement are evaluated left to right, unless parentheses dictate otherwise. INTERSECT binds more tightly than UNION. That is, A UNION B INTERSECT C will be read as A UNION (B INTERSECT C).

Currently, FOR UPDATE and FOR SHARE may not be specified either for an INTERSECT result or for any input of an INTERSECT.

The EXCEPT Clause

The EXCEPT clause has this general form:

<select_statement> EXCEPT [ALL] <select_statement>

where <select_statement> is any SELECT statement without an ORDER BY, LIMIT, FOR UPDATE, or FOR SHARE clause.

The EXCEPT operator computes the set of rows that are in the result of the left SELECT statement but not in the result of the right one.

The result of EXCEPT does not contain any duplicate rows unless the ALL option is specified. With ALL, a row that has m duplicates in the left table and n duplicates in the right table will appear max(m-n,0) times in the result set.

Multiple EXCEPT operators in the same SELECT statement are evaluated left to right unless parentheses dictate otherwise. EXCEPT binds at the same level as UNION.

Currently, FOR UPDATE and FOR SHARE may not be specified either for an EXCEPT result or for any input of an EXCEPT.

The ORDER BY Clause

The optional ORDER BY clause has this general form:

ORDER BY <expression> [ASC | DESC | USING <operator>] [, ...]

where <expression> can be the name or ordinal number of an output column (SELECT list item), or it can be an arbitrary expression formed from input-column values.

The ORDER BY clause causes the result rows to be sorted according to the specified expressions. If two rows are equal according to the left-most 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.

The ordinal number refers to the ordinal (left-to-right) position of the result column. This feature makes it possible to define an ordering on the basis of a column that does not have a unique name. This is never absolutely necessary because it is always possible to assign a name to a result column using the AS clause.

It is also possible to use arbitrary expressions in the ORDER BY clause, including columns that do not appear in the SELECT result list. Thus the following statement is valid:

SELECT name FROM distributors ORDER BY code;

A limitation of this feature is that an ORDER BY clause applying to the result of a UNION, INTERSECT, or EXCEPT clause may only specify an output column name or number, not an expression.

If an ORDER BY expression is a simple name that matches both a result column name and an input column name, ORDER BY will interpret it as the result column name. This is the opposite of the choice that GROUP BY will make in the same situation. This inconsistency is made to be compatible with the SQL standard.

Optionally one may add the key word ASC (ascending) or DESC (descending) after any expression in the ORDER BY clause. If not specified, ASC is assumed by default. Alternatively, a specific ordering operator name may be specified in the USING clause. ASC is usually equivalent to USING < and DESC is usually equivalent to USING >. (But the creator of a user-defined data type can define exactly what the default sort ordering is, and it might correspond to operators with other names.)

The null value sorts higher than any other value. In other words, with ascending sort order, null values sort at the end, and with descending sort order, null values sort at the beginning.

Character-string data is sorted according to the locale-specific collation order that was established when the HAWQ system was initialized.

The DISTINCT Clause

If DISTINCT is specified, all duplicate rows are removed from the result set (one row is kept from each group of duplicates). ALL specifies the opposite: all rows are kept. ALL is the default.

DISTINCT ON ( <expression> [, ...] ) keeps only the first row of each set of rows where the given expressions evaluate to equal. The DISTINCT ON expressions are interpreted using the same rules as for ORDER BY. Note that the ‘first row’ of each set is unpredictable unless ORDER BY is used to ensure that the desired row appears first. For example:

SELECT DISTINCT ON (location) location, time, report FROM
weather_reports ORDER BY location, time DESC;

retrieves the most recent weather report for each location. But if we had not used ORDER BY to force descending order of time values for each location, we would have gotten a report from an unpredictable time for each location.

The DISTINCT ON expression(s) must match the left-most ORDER BY expression(s). The ORDER BY clause will normally contain additional expression(s) that determine the desired precedence of rows within each DISTINCT ON group.

The LIMIT Clause

The LIMIT clause consists of two independent sub-clauses:

LIMIT {<count> | ALL}
OFFSET <start>

where <count> specifies the maximum number of rows to return, while <start> specifies the number of rows to skip before starting to return rows. When both are specified, start rows are skipped before starting to count the count rows to be returned.

When using LIMIT, it is a good idea to use an ORDER BY clause that constrains the result rows into a unique order. Otherwise you will get an unpredictable subset of the query’s rows. You may be asking for the tenth through twentieth rows, but tenth through twentieth in what ordering? You don’t know what ordering unless you specify ORDER BY.

The query planner takes LIMIT into account when generating a query plan, so you are very likely to get different plans (yielding different row orders) depending on what you use for LIMIT and OFFSET. Thus, using different LIMIT/OFFSET values to select different subsets of a query result will give inconsistent results unless you enforce a predictable result ordering with ORDER BY. This is not a defect; it is an inherent consequence of the fact that SQL does not promise to deliver the results of a query in any particular order unless ORDER BY is used to constrain the order.

Examples

To join the table films with the table distributors:

SELECT f.title, f.did, d.name, f.date_prod, f.kind FROM
distributors d, films f WHERE f.did = d.did

To sum the column length of all films and group the results by kind:

SELECT kind, sum(length) AS total FROM films GROUP BY kind;

To sum the column length of all films, group the results by kind and show those group totals that are less than 5 hours:

SELECT kind, sum(length) AS total FROM films GROUP BY kind
HAVING sum(length) < interval '5 hours';

Calculate the subtotals and grand totals of all sales for movie kind and distributor.

SELECT kind, distributor, sum(prc*qty) FROM sales
GROUP BY ROLLUP(kind, distributor)
ORDER BY 1,2,3;

Calculate the rank of movie distributors based on total sales:

SELECT distributor, sum(prc*qty),
       rank() OVER (ORDER BY sum(prc*qty) DESC)
FROM sale
GROUP BY distributor ORDER BY 2 DESC;

The following two examples are identical ways of sorting the individual results according to the contents of the second column (name):

SELECT * FROM distributors ORDER BY name;
SELECT * FROM distributors ORDER BY 2;

The next example shows how to obtain the union of the tables distributors and actors, restricting the results to those that begin with the letter W in each table. Only distinct rows are wanted, so the key word ALL is omitted:

SELECT distributors.name FROM distributors WHERE
distributors.name LIKE 'W%' UNION SELECT actors.name FROM
actors WHERE actors.name LIKE 'W%';

This example shows how to use a function in the FROM clause, both with and without a column definition list:

CREATE FUNCTION distributors(int) RETURNS SETOF distributors
AS $$ SELECT * FROM distributors WHERE did = $1; $$ LANGUAGE
SQL;
SELECT * FROM distributors(111);

CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS
$$ SELECT * FROM distributors WHERE did = $1; $$ LANGUAGE
SQL;
SELECT * FROM distributors_2(111) AS (dist_id int, dist_name
text);

Compatibility

The SELECT statement is compatible with the SQL standard, but there are some extensions and some missing features.

Omitted FROM Clauses

HAWQ allows you to omit the FROM clause. It has a straightforward use to compute the results of simple expressions. For example:

SELECT 2+2;

Some other SQL databases cannot do this except by introducing a dummy one-row table from which to do the SELECT.

Note that if a FROM clause is not specified, the query cannot reference any database tables. For compatibility with applications that rely on this behavior the add_missing_from configuration parameter can be enabled.

The AS Key Word

In the SQL standard, the optional key word AS is just noise and can be omitted without affecting the meaning. The HAWQ parser requires this key word when renaming output columns because the type extensibility features lead to parsing ambiguities without it. AS is optional in FROM items, however.

Namespace Available to GROUP BY and ORDER BY

In the SQL-92 standard, an ORDER BY clause may only use result column names or numbers, while a GROUP BY clause may only use expressions based on input column names. HAWQ extends each of these clauses to allow the other choice as well (but it uses the standard’s interpretation if there is ambiguity). HAWQ also allows both clauses to specify arbitrary expressions. Note that names appearing in an expression will always be taken as input-column names, not as result-column names.

SQL:1999 and later use a slightly different definition which is not entirely upward compatible with SQL-92. In most cases, however, HAWQ will interpret an ORDER BY or GROUP BY expression the same way SQL:1999 does.

Nonstandard Clauses

The clauses DISTINCT ON, LIMIT, and OFFSET are not defined in the SQL standard.

Limited Use of STABLE and VOLATILE Functions

To prevent data from becoming out-of-sync across the segments in HAWQ, any function classified as STABLE or VOLATILE cannot be executed at the segment database level if it contains SQL or modifies the database in any way.

See Also

EXPLAIN