The

CommentsProcedural SQL (PSQL) Statements

The `SELECT` Columns List

The columns list contains one or more comma-separated value expressions.Each expression provides a value for one output column.Alternatively, * (“star” or “all”) can be used to stand for all the columns of all relations in the FROM clause.

Syntax

SELECT
  [...]
  [{ ALL | DISTINCT }] <select-list>
  [...]
  FROM ...

<select_list> ::= * | <select-sublist> [, <select-sublist> ...]

<select-sublist> ::=
    table-alias.*
  | <value-expression> [[AS] column-alias]

<value-expression> ::=
    [table-alias.]col_name
  | [table-alias.]selectable_SP_outparm
  | <literal>
  | <context-variable>
  | <function-call>
  | <single-value-subselect>
  | <CASE-construct>
  | any other expression returning a single
    value of a Firebird data type or NULL

<function-call> ::=
    <normal-function>
  | <aggregate-function>
  | <window-function>

<normal-function> ::=
  !! See Built-in Scalar Functions !!

<aggregate-function> ::=
  !! See Aggregate Functions !!

<window-function> ::=
  !! See Window Functions !!

Table 1. Arguments for the `SELECT` Columns List
Argument	Description
table-alias	Name of relation (view, stored procedure, derived table), or its alias
col_name	Name of a table or view column, or its alias
selectable_SP_outparm	Declared name of an output parameter of a selectable stored procedure
literal	A literal
context-variable	Context variable
function-call	Scalar, aggregate, or window function expression
single-value-subselect	A subquery returning one scalar value (singleton)
CASE-construct	CASE construct setting conditions for a return value

It is always valid to qualify a column name (or “*”) with the name or alias of the table, view or selectable SP to which it belongs, followed by a dot (‘.’).For example, relationname.columnname, relationname.*, alias.columnname, alias.*.Qualifying is required if the column name occurs in more than one relation taking part in a join.Qualifying “*” is required if it is not the only item in the column list.

Important

Aliases hide the original relation name: once a table, view or procedure has been aliased, only the alias can be used as its qualifier throughout the query.The relation name itself becomes unavailable.

The column list may optionally be preceded by one of the keywords DISTINCT or ALL:

DISTINCT filters out any duplicate rows.That is, if two or more rows have the same values in every corresponding column, only one of them is included in the result set
ALL is the default: it returns all rows, including duplicates.ALL is rarely used;it is allowed for compliance with the SQL standard.

A COLLATE clause of a value-expression will not change the appearance of the column as such.However, if the specified collation changes the case or accent sensitivity of the column, it may influence:

The ordering, if an ORDER BY clause is also present, and it involves that column
Grouping, if the column is part of a GROUP BY clause
The rows retrieved (and hence the total number of rows in the result set), if DISTINCT is used

Examples of `SELECT` queries with different types of column lists

A simple SELECT using only column names:

select cust_id, cust_name, phone
  from customers
  where city = 'London'

A query featuring a concatenation expression and a function call in the columns list:

select 'Mr./Mrs. ' || lastname, street, zip, upper(city)
  from contacts
  where date_last_purchase(id) = current_date

A query with two subselects:

select p.fullname,
  (select name from classes c where c.id = p.class) as class,
  (select name from mentors m where m.id = p.mentor) as mentor
from pupils p

The following query accomplishes the same as the previous one using joins instead of subselects:

select p.fullname,
  c.name as class,
  m.name as mentor
  join classes c on c.id = p.class
from pupils p
  join mentors m on m.id = p.mentor

This query uses a CASE construct to determine the correct title, e.g. when sending mail to a person:

select case upper(sex)
    when 'F' then 'Mrs.'
    when 'M' then 'Mr.'
    else ''
  end as title,
  lastname,
  address
from employees

Query using a window function, ranks employees by salary.

SELECT
  id,
  salary,
  name ,
  DENSE_RANK() OVER (ORDER BY salary) AS EMP_RANK
FROM employees
ORDER BY salary;

Querying a selectable stored procedure:

select * from interesting_transactions(2010, 3, 'S')
  order by amount

Selecting from columns of a derived table.A derived table is a parenthesized SELECT statement whose result set is used in an enclosing query as if it were a regular table or view.The derived table is shown in bold here:

select fieldcount,
  count(relation) as num_tables
from (select r.rdb$relation_name as relation,
        count(*) as fieldcount
      from rdb$relations r
        join rdb$relation_fields rf
          on rf.rdb$relation_name = r.rdb$relation_name
      group by relation)
group by fieldcount

Asking the time through a context variable (CURRENT_TIME):

select current_time from rdb$database

For those not familiar with RDB$DATABASE: this is a system table that is present in all Firebird databases and is guaranteed to contain exactly one row.Although it wasn’t created for this purpose, it has become standard practice among Firebird programmers to select from this table if you want to select “from nothing”, i.e. if you need data that are not bound to a table or view, but can be derived from the expressions in the output columns alone.Another example is:

select power(12, 2) as twelve_squared, power(12, 3) as twelve_cubed
  from rdb$database

Finally, an example where you select some meaningful information from RDB$DATABASE itself:

select rdb$character_set_name from rdb$database

As you may have guessed, this will give you the default character set of the database.

The

The SELECT Columns List

Examples of SELECT queries with different types of column lists

The `SELECT` Columns List

Examples of `SELECT` queries with different types of column lists