Select List 

The select list is used to specify expressions and display names for the columns of the result table of a query specification.

Although syntactically this clause is the first clause of a query specification, it is evaluated last.

 

Syntax

<select list> ::= '*'                 | <derived column> ( ',' <derived column> )*. <derived column> ::= <value expression>                      ( ( AS )? <column alias>)?. <display name> ::= the name of the column in the result set; see below

 

The <select list> can be applied to a grouped table (the result of GROUP BY or HAVING  clause), or an ungrouped table (the result of FROM or WHERE clause).

If the <select list> contains a <set function> and the table, to which it is applied, is not already grouped (that is, neither a GROUP BY clause nor a HAVING clause has been applied), the table is implicitly grouped. The result is a grouped table with just one group that has no grouping columns.

If the <select list> is applied to a grouped table, then the column references in the <select list> must either reference a grouping column, or must be specified inside a <set function>. The cardinality of the result table is the number of groups in the grouped table. If the <select list> contains a <set function>, all columns that are not aggregated must be listed in the GROUP BY clause.

If the <select list> is applied to an ungrouped table, the cardinality of the result table is the number of rows in the ungrouped table.

If the <select list> is '*', the result table is the table to which the <select list> is applied. For every column in the result set, the <display name> is the <column name> of the <column reference> underlying the result set column.

If the <select list> is not '*', the <display name> of the result set columns is determined as follows:

●      If a <column alias> is specified, the <display name> is that <column alias>,

●      else,

○       if the <value expression> is a <column reference>, then the <display name> is the <column name> of the <column reference>.

○       Otherwise, the <display name> is undefined.

The result of a <select list> is a table with the same degree as the <select list>. Each <value expression> is evaluated for every row respective group of the table and the result row consists of the sequence of these values.

Examples

SELECT * FROM employees

SELECT *.  This query selects all columns from all rows of the table employees.

SELECT employee_name AS name, salary FROM employees

Derived Column List.  This query selects the name and the salary for all employees.

More Information

Value expressions

GROUP BY Clause

HAVING Clause