sql: sub-queries single-value sub-queries single-column sub-queries sub-queries that produce tables...

SQL: Sub-queries

Single-value sub-queriesSingle-column sub-queries

Sub-queries that produce tablesCorrelated sub-queries

D. Christozov / G.Tuparov INF 280 Database Systems: SQL - Sub queries 1

General notes (1)Sub-query

Queries used to produce values for processing within other statements (basically in filter-conditions WHERE or HAVING, and in FROM to join with other tables or as views for sequential query processing);

Result of a sub-query are:– Tables may be used in FROM clause or with

EXISTS / NOT EXISTS predicates; – Multivalued - the table has a single column;– Single row – may be used in row value constructor

(with INSERT statement);– Scalar or single-valued - may be used in comparisons

D. Christozov / G.TuparovINF 280 Database Systems:

SQL – Sub-queries 2

General notes (2) -> General Rules• The entire sub-query must always be enclosed in

parentheses;• The ORDER BY clause may not be used in a sub-query;• Sub-queries may contain nested sub-queries: SQL evaluates

them from inside out;• When in doubt - qualify! Explicit qualification of column

names is necessary only when referring to a table listed in FROM at a higher level query, but it is always permissible to qualify columns names to clarify the meaning;

• VISIBILITY: A sub-query may refer only to column names in tables listed in its FROM clause and the FROM clauses in outer (parent) queries. A sub-query may not access tables used only by a child query;



General notes (3) -> General rules (cont.)• When a sub-query is one of the two operands involved in a

comparison, the sub-query must be the second one -- it must follow the relational operator.

• The result of a sub-query can not be directly compared with the result of another sub-query;

• Sub-queries may not be used as operands in expressions;• If you hesitate between using sub-query and join:

– when the final result draws information from more than one table, use join;

– when the final result involves information from only one of the tables -- use a sub-query (in general it is cheaper than join);

– when necessary - combine.



Fancy Fruits Database


SQL - Single Table queries 5

Single-valued sub-query (1)• It may be used with relational operators just as if it

is a constant.Example: Search test for equality on the unique attribute (Fancy Fruits DB/MySQL 5.6)SELECT *FROM OrdersWHERE Item_id = (SELECT Item_id FROM Stock WHERE descript = ‘KIWI’);



Single-valued sub-query (2)Example: Use of aggregate functions (Fancy Fruits DB)

SELECT *FROM VendorsWHERE cost < (SELECT avg(cost) FROM Vendors WHERE Item_id = ‘I02’);



Oracle 11g

MySQL 5.6

Multi-valued sub-query (single column)• Use with linking (quantified) predicates ANY, SOME,

ALL– SOME and ANY are equivalent, they link a simple

relational operator with the sub-query. The relational operator is applied to each row of the result of the sub-query and the logical expression is true if and only if one or more (at least one) rows in the sub-query result satisfy the comparison.

– ALL is true if and only if the relevant comparison is true for each and every value in the sub-query result. Be careful when use ALL with relational operator equal!



Multi-valued sub-query (single column)• Multi-valued sub-query (continue)

– Use with list/set of values: IN and NOT INx IN (a,b,c) x = SOME (a, b, c)x NOT IN (a,b,c) x <> ALL (a, b, c)– Use with EXISTS and NOT EXISTS EXISTS is true if and

only if there exists at least one row in the sub-query result and the result rows may have more than one column.

• Tables are used only with EXISTS and NOT EXISTS and are useful only as correlated sub-queries

• Multiple queries– Multiple sub-queries on the same level: used with compound

logical expressions – Nested sub-queries



Examples (Fancy Fruits DB/MySQL 5.6)Multi-valued (single-column) queries: ANY

SELECT DISTINCT Cust_idFROM OrdersWHERE quantity > ANY

(SELECT quantity FROM Orders WHERE Cust_id = ‘CCC’);

SOMESELECT DISTINCT Cust_idFROM OrdersWHERE quantity > SOME




Examples (Fancy Fruits DB)Multi-valued (single-column) queries:

ALLSELECT DISTINCT Cust_idFROM OrdersWHERE quantity > ALL




Oracle 11g

MySQL 5.6

Examples (Fancy Fruits DB)Multiple sub-queries -> On the same level

SELECT *FROM OrdersWHERE Cust_id =

(SELECT Cust_id FROM Customers WHERE cust_name = ‘Alice’ )

AND Item_id = (SELECT Item_id FROM Stock WHERE descript = ‘Oranges’ );



Oracle 11g

MySQL 5.6

Examples (Fancy Fruits DB)Multiple sub-queries -> Nested

SELECT Vendor_idFROM VendorsWHERE Item_id in ( SELECT Item_id FROM Orders

WHERE Cust_id = ( SELECT Cust_id

FROM Customers WHERE Cust_name = ‘Alice’ ) );



Oracle 11gMySQL 5.6

Correlated sub-queries• Some sub-queries are independent of their outer

(parent) query: they return the same result any time they are evaluated.

• The result of other sub-queries, correlated, depends on the row being processed by the outer query:– the sub-query result varies with the row being processed

by the outer query;– the sub-query must be re-evaluated for each row of the

outer query;– processing time of correlated sub-queries is theoretically

equal of joins, but practically much higher (when join combines tables according to fields, which are indexed).



Examples (Fancy Fruits DB/MySQL 5.6)Correlated sub-queries (1)SELECT *FROM Orders C1WHERE C1.quantity >

(SELECT AVG(quantity) FROM Orders C2 WHERE C2.Cust_id = C1.Cust_id );



C1.Cust-id == ‘AAA’Subquery: SELECT AVG(Quantity) produces: (10 + 30 + 60 + 70 )/ 4.0 = 42.5

FROM Orders C2 10 > 42.5 FALSEWHERE C2.Cust_id = ‘AAA’ the first row is not included into result

C1.Cust-id == ‘BBB’Subquery: SELECT AVG(Quantity) produces: (20 + 50) / 2.0 = 35

FROM Orders C2 20 > 35 FALSEWHERE C2.Cust_id = ‘BBB’ the second row is not included into result

Examples (Fancy Fruits DB/MySQL 5.6)Correlated sub-queries (2)

SELECT *FROM Orders C1WHERE C1.quantity <

(SELECT AVG(quantity)FROM Orders C2WHERE C2.Cust_id = C1.Cust_id);



Examples (Fancy Fruits DB)Correlated sub-queries (3)

SELECT cust_nameFROM CustomersWHERE 100 <

(SELECT SUM(quantity)FROM OrdersWHERE Orders.Cust_id = Customers.Cust_id );



Examples (Fancy Fruits DB)Correlated sub-queries (4)

SELECT Item_idFROM OrdersGROUP BY Item_idHAVING sum(quantity) >

(SELECT 0.1*on_handFROM StockWHERE Orders.Item_id = Stock.Item_id);



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