Question

I often find myself in need of very complex SQL examples when testing abstraction concepts or just comparing database styles and structures while working with PostgreSQL, MySQL, and even SQLite.

I assume that means there are others in need of insane queries to open our eyes to what is possible and insure our DB layers can handle anything we throw at them.

So, can anyone share some queries that would through even the most hardcore, ORM-all-the-way guy for a loop?

PostgreSQL

SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
    * | expression [ [ AS ] output_name ] [, ...]
    [ FROM from_item [, ...] ]
    [ WHERE condition ]
    [ GROUP BY expression [, ...] ]
    [ HAVING condition [, ...] ]
    [ WINDOW window_name AS ( window_definition ) [, ...] ]
    [ { UNION | INTERSECT | EXCEPT } [ ALL ] select ]
    [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
    [ LIMIT { count | ALL } ]
    [ OFFSET start [ ROW | ROWS ] ]
    [ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
    [ FOR { UPDATE | SHARE } [ OF table_name [, ...] ] [ NOWAIT ] [...] ]

MySQL

SELECT
    [ALL | DISTINCT | DISTINCTROW ]
      [HIGH_PRIORITY]
      [STRAIGHT_JOIN]
      [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
      [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
    select_expr [, select_expr ...]
    [FROM table_references
    [WHERE where_condition]
    [GROUP BY {col_name | expr | position}
      [ASC | DESC], ... [WITH ROLLUP]]
    [HAVING where_condition]
    [ORDER BY {col_name | expr | position}
      [ASC | DESC], ...]
    [LIMIT {[offset,] row_count | row_count OFFSET offset}]
    [PROCEDURE procedure_name(argument_list)]
    [INTO OUTFILE 'file_name'
        [CHARACTER SET charset_name]
        export_options
      | INTO DUMPFILE 'file_name'
      | INTO var_name [, var_name]]
    [FOR UPDATE | LOCK IN SHARE MODE]]

Answer 1

Check out this talk during OpenSQLCamp, November 2009.

Title: SQL For the Insane

Also
Recursion with SQL

The Problem

Suppose you are tracking supplies and have a field called si_item and another called si_parentid. The parent keeps track of what subclass a supply item belongs to. E.g. you have paper parent that has subclasses such as recycled, non-recycled. When someone takes supplies, you want to return the fully qualified name e.g. Paper->Recycled->20 Lb

Solution

CREATE TABLE supplyitem(si_id integer PRIMARY KEY, si_parentid integer, si_item varchar(100));

--load up the table (multirow constructor introduced in 8.2)
INSERT INTO supplyitem(si_id,si_parentid, si_item)
VALUES (1, NULL, 'Paper'),
(2,1, 'Recycled'),
(3,2, '20 lb'),
(4,2, '40 lb'),
(5,1, 'Non-Recycled'),
(6,5, '20 lb'),
(7,5, '40 lb'),
(8,5, 'Scraps');

--Recursive query (introduced in 8.4 returns fully qualified name)
WITH RECURSIVE supplytree AS
(SELECT si_id, si_item, si_parentid, CAST(si_item As varchar(1000)) As si_item_fullname
FROM supplyitem
WHERE si_parentid IS NULL
UNION ALL
SELECT si.si_id,si.si_item,
    si.si_parentid,
    CAST(sp.si_item_fullname || '->' || si.si_item As varchar(1000)) As si_item_fullname
FROM supplyitem As si
    INNER JOIN supplytree AS sp
    ON (si.si_parentid = sp.si_id)
)
SELECT si_id, si_item_fullname
FROM supplytree
ORDER BY si_item_fullname;

Result looks like

si_id |      si_item_fullname
------+-----------------------------
 1    | Paper
 5    | Paper->Non-Recycled
 6    | Paper->Non-Recycled->20 lb
 7    | Paper->Non-Recycled->40 lb
 8    | Paper->Non-Recycled->Scraps
 2    | Paper->Recycled
 3    | Paper->Recycled->20 lb
 4    | Paper->Recycled->40 lb