Examples Of

CommentsProcedural SQL (PSQL) Statements

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 `FROM` clause

The FROM clause specifies the source(s) from which the data are to be retrieved.In its simplest form, this is a single table or view.However, the source can also be a selectable stored procedure, a derived table, or a common table expression.Multiple sources can be combined using various types of joins.

This section focuses on single-source selects.Joins are discussed in a following section.

Syntax

SELECT
  ...
  FROM <table-reference> [, <table-reference> ...]
  [...]

<table-reference> ::= <table-primary> | <joined-table>

<table-primary> ::=
    <table-or-query-name> [[AS] correlation-name]
  | [LATERAL] <derived-table> [<correlation-or-recognition>]
  | <parenthesized-joined-table>

<table-or-query-name> ::=
    table-name
  | query-name
  | [package-name.]procedure-name [(<procedure-args>)]

<procedure-args> ::= <value-expression [, <value-expression> ...]

<derived-table> ::= (<query-expression>)

<correlation-or-recognition> ::=
  [AS] correlation-name [(<column-name-list>)]

<column-name-list> ::= column-name [, column-name ...]

Table 1. Arguments for the `FROM` Clause
Argument	Description
table-name	Name of a table or view
query-name	Name of a CTE
package-name	Name of a package
procedure-name	Name of a selectable stored procedure
procedure-args	Selectable stored procedure arguments
derived-table	Derived table query expression
correlation-name	The alias of a data source (table, view, procedure, CTE, derived table)
column-name	Name or alias for a column in a relation, CTE or derived table

Examples Of

Examples of SELECT queries with different types of column lists

The FROM clause

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

The `FROM` clause