Common table expressions (CTEs)

Common table expressions or CTEs allow a complex subquery to be represented in a short way later on for improved readability, and reuse multiple times in a query.

CTEs do not affect query performance.

Syntax

cte ::=
    [ WITH table_alias AS (query)
         [, ...]
    ]


query ::=
   query_term
   [ UNION ALL query_term [ ...] ]
   [ ORDER BY order [, ... ] ]
   [ LIMIT num_rows ]
   ;

query_term ::=
      SELECT
        [ TOP num_rows ]
        [ DISTINCT ]
        select_list
        [ FROM table_ref [, ... ]
            [ WHERE value_expr
            [ GROUP BY value_expr [, ... ]
               [ HAVING value_expr ]
            ]
      | ( VALUES ( value_expr [, ... ] ) [, ... ] )

Examples

Simple CTE

nba=> WITH s AS (SELECT "Name" FROM nba WHERE "Salary" > 20000000)
.        SELECT * FROM nba AS n, s WHERE n."Name" = s."Name";
Name            | Team                  | Number | Position | Age | Height | Weight | College      | Salary   | name0
----------------+-----------------------+--------+----------+-----+--------+--------+--------------+----------+----------------
Carmelo Anthony | New York Knicks       |      7 | SF       |  32 | 6-8    |    240 | Syracuse     | 22875000 | Carmelo Anthony
Chris Bosh      | Miami Heat            |      1 | PF       |  32 | 6-11   |    235 | Georgia Tech | 22192730 | Chris Bosh
Chris Paul      | Los Angeles Clippers  |      3 | PG       |  31 | 6-0    |    175 | Wake Forest  | 21468695 | Chris Paul
Derrick Rose    | Chicago Bulls         |      1 | PG       |  27 | 6-3    |    190 | Memphis      | 20093064 | Derrick Rose
Dwight Howard   | Houston Rockets       |     12 | C        |  30 | 6-11   |    265 |              | 22359364 | Dwight Howard
Kevin Durant    | Oklahoma City Thunder |     35 | SF       |  27 | 6-9    |    240 | Texas        | 20158622 | Kevin Durant
Kobe Bryant     | Los Angeles Lakers    |     24 | SF       |  37 | 6-6    |    212 |              | 25000000 | Kobe Bryant
LeBron James    | Cleveland Cavaliers   |     23 | SF       |  31 | 6-8    |    250 |              | 22970500 | LeBron James

In this example, the WITH clause defines the temporary name r for the subquery which finds salaries over $20 million. The result set becomes a valid table reference in any table expression of the subsequent SELECT clause.

Nested CTEs

SQream DB also supports any amount of nested CTEs, such as this:

WITH w AS
    (SELECT * FROM
        (WITH x AS (SELECT * FROM nba) SELECT * FROM x ORDER BY "Salary" DESC))
  SELECT * FROM w ORDER BY "Weight" DESC;

Reusing CTEs

SQream DB supports reusing CTEs several times in a query:

nba=> WITH
.        nba_ct AS (SELECT "Name", "Team" FROM nba WHERE "College"='Connecticut'),
.        nba_az AS (SELECT "Name", "Team" FROM nba WHERE "College"='Arizona')
.        SELECT * FROM nba_az JOIN nba_ct ON nba_ct."Team" = nba_az."Team";
Name            | Team            | name0          | team0
----------------+-----------------+----------------+----------------
Stanley Johnson | Detroit Pistons | Andre Drummond | Detroit Pistons
Aaron Gordon    | Orlando Magic   | Shabazz Napier | Orlando Magic