Merging Multiple QueriesPertemuan 6

Matakuliah : T0413/Current Popular IT IITahun : 2007

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AGENDA: • UNION • UNION with ORDER BY • INTERSECT • EXCEPT or MINUS • Examples

Book:Mastering SQL by Martin GruberSybex (2000)Chapter : 13

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UNION

• Uniting multiple queries as one output result• Using UNION clause• SELECT snum, sname FROM Salespeople

WHERE city = ‘London’UNIONSELECT cnum, cname FROM Customers

WHERE city = ‘London’

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UNION (cont’d)

• The columns selected by the two statements are outputs as though they were one.

• UNION will automatically eliminate duplicate rows from the output.

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UNION (cont’d)• Using Strings and Expression with UNION• For example if you want to create a report:

SELECT a.snum, sname, onum, ‘Highest on’, odateFROM Salespeople a, Orders bWHERE a.snum = b.snumAND b.amt =(SELECT MAX(amt) FROM Orders cWHERE c.odate = b.odate)

UNIONSELECT a.snum, sname, onum, ‘Lowest on’, odate

FROM Salespeople a, Orders bWHERE a.snum = b.snumAND b.amt =

(SELECT MIN(amt) FROM Orders cWHERE c.odate = b.odate)

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UNION with ORDER BY• The data from the multiple queries are not output with any

particular order.• To order the output from a UNION, we can use ORDER BY.

SELECT a.snum, sname, onum, ‘Highest on’, odateFROM Salespeople a, Orders bWHERE a.snum = b.snumAND b.amt =(SELECT MAX(amt) FROM Orders cWHERE c.odate = b.odate)

UNIONSELECT a.snum, sname, onum, ‘Lowest on’, odate

FROM Salespeople a, Orders bWHERE a.snum = b.snumAND b.amt =

(SELECT MIN(amt) FROM Orders cWHERE c.odate = b.odate)

ORDER BY 3;

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INTERSECT• The INTERSECT operator finds the intersection of rows output by

the two or more queries.• Example:

SELECT snumFROM Orders aWHERE 1500.00 <(SELECT SUM(amt) FROM Orders b

WHERE b.snum = a.snum)INTERSECTSELECT snum

FROM Salespeople cWHERE 2 >(SELECT COUNT(*) FROM Customers d

WHERE d.snum = c.snum);– The query above try to find which salesperson had more than $1500.00

total in current orders with less than two customers assigned.

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EXCEPT or MINUS

• The main idea of EXEPT is it takes two queries, A and B, and includes in the output only the rows from A that were not also produced by B.

• In other words, it excludes rows produced by B but not by A, so the effect is that no rows output by query B are in the final output in any case.

• EXCEPT is respective of the order in which queries are stated.

• So the output will be different if the order of the queries are being switched.

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Examples• SELECT snum

FROM Orders aWHERE 1500.00 <(SELECT SUM(amt) FROM Orders b

WHERE b.snum = a.snum)EXCEPTSELECT snum

FROM Salespeople cWHERE 2 >(SELECT COUNT(*) FROM Customers d

WHERE d.snum = c.snum);

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Examples (cont’d)• SELECT snum

FROM Salespeople cWHERE 2 >(SELECT COUNT(*) FROM Customers d

WHERE d.snum = c.snum);EXCEPTSELECT snum

FROM Orders aWHERE 1500.00 <(SELECT SUM(amt) FROM Orders b

WHERE b.snum = a.snum)

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Examples (cont’d)• The output of previous two queries are different,

because the order of the queries using EXCEPT were switched.

• The first query will produce output : Snum = 1001 and 1002

• While the second query will produce output : Snum = 1007 and 1020

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End ofMerging Multiple Queries

Thank you