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Absolute Database SQL Engine supports correlated and non-correlated subqueries in SELECT, UPDATE and DELETE statements.
You can use the following types of subqueris:
Nested queries in the SELECT FROM clause
Usually SELECT uses tables to get records, but it is possible to use a subquery instead of a table.
Example:
| /* The subquery retrieves count of unique names in table "Employee", then function "count" counts quantity of these names.*/
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| SELECT COUNT(*) UniqueNamesCount FROM (SELECT DISTINCT FirstName FROM Employee)
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Subqueries in the WHERE clause
A subquery works like a search condition to restrict the number of rows returned by the outer, or "parent" query.
Examples:
| /* The query returns all venues where no event will take place. */
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| SELECT Venue FROM venues v WHERE NOT EXISTS (SELECT * FROM events WHERE VenueNo=v.VenueNo)
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| /* The query returns only those venues where some event will occur.*/
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| SELECT Venue FROM venues WHERE VenueNo IN (SELECT VenueNo FROM events)
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| /* This query returns events that will occur on venues with capacity lower than in venues 3 and 4.*/
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| SELECT e.Event_Name, v.Capacity FROM events e, venues v WHERE e.VenueNo=v.VenueNo AND
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| v.Capacity < ALL (SELECT Capacity FROM venues WHERE VenueNo BETWEEN 3 AND 4)
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| /*This query returns events that will occur on venues with capacity more than in venue 3 or 4.*/
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| SELECT e.Event_Name, v.Capacity FROM events e, venues v WHERE e.VenueNo=v.VenueNo AND
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| v.Capacity > ANY (SELECT Capacity FROM venues WHERE VenueNo BETWEEN 3 AND 4)
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| /* This query shows all venues where only one event will occur.*/
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| SELECT Venue FROM venues v WHERE 1 = (SELECT COUNT(*) FROM events WHERE VenueNo=v.VenueNo)
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Subqueries in the SELECT fields list
A subquery works like an advanced expression.
Example:
| /* For each entry in table "Venues" the query counts quantity of events from table "Events" and places this number in a column named "EventsCount".*/
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| SELECT Venue, (SELECT COUNT(*) FROM Events WHERE VenueNo = v.VenueNo) EventsCount FROM Venues v ORDER BY EventsCount DESC
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Subqueries in the UPDATE statement
A subquery works like an advanced expression allowing to assign data from another table.
Examples:
| /* The query works as follows. First, only those entries are picked that have "Ticket_price" 5 of less. Then corresponding venue is taken, its capacity is multiplied by two and set as the new ticket price. Notice that NULL fields are not affected.*/
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| UPDATE Events e SET Ticket_price = (SELECT Capacity FROM Venues WHERE VenueNo=e.VenueNo)*2 WHERE Ticket_price <= 5
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| UPDATE Orders SET (ShipToAddr1,ShipToAddr2) = (SELECT Addr1, Addr2 FROM Customer WHERE CustNo=Orders.CustNo) WHERE CustNo IN (1221, 2156)
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