Temporary SQL Tables Los Angeles Pierce College Computer Science 560

How Do We Get One Column From Two Tables? NCAA Table: –NCAA_SEQ_KEY NOT NULL NUMBER(5) –NCAA_YEAR NUMBER(4) –NCAA_PLACE NUMBER(1) –NCAA_SCHOOL CHAR(30) NIT Table: –NIT_SEQ_KEY NOT NULL NUMBER(5) –NIT_YEAR NUMBER(4) –NIT_PLACE NUMBER(1) –NIT_SCHOOL CHAR(30) –NIT_TIE CHAR(1)

What’s The Desired Output? Three columns: –School Name –Tournament won –Year Something like: School Tournament Year UCLA NCAA 1972 UCLA NIT 1985 UCLA NCAA 1995

The Problem How do we get two different columns from two different tables into one result column? (School, year) 1 How do we insert the table name into a result column? Answer: cheat. 1 We’ll pretend we can’t do this with UNION

Temporary SQL Tables We really want a table which looks like: CREATE TABLE BB_WINNERS ( BB_SEQUENCE NUMBER(5) CONSTRAINT bb_seq_pk PRIMARY KEY, SCHOOL CHAR(30), YEAR NUMBER(4), TOURNAMENT CHAR(4), PLACE NUMBER(1), TIE CHAR(1));

Ideal SQL Selection Statement SELECT SCHOOL,TOURNAMENT, YEAR FROM BB_WINNERS WHERE PLACE = 1 ORDER BY SCHOOL, YEAR;

Drawbacks To The Ideal Case A special table for one query is probably wasteful Additional table space needed for something which is rarely used Data is duplicated – not an issue in our sample case, but potentially a problem in a real world situation

Solution: Temporary SQL Tables Acts just like a normal table BUT – only exists for the duration of a session or a transaction Disk space for table only needed when it is actually in use Note: assume we have a sequence called BB_SEQ already defined.

Creating A Temporary Table CREATE GLOBAL TEMPORARY TABLE BB_WINNERS ( BB_SEQUENCE NUMBER(5) CONSTRAINT bb_seq_pk PRIMARY KEY, SCHOOL CHAR(30), YEAR NUMBER(4), TOURNAMENT CHAR(4), PLACE NUMBER(1), TIE CHAR(1)) ON COMMIT PRESERVE ROWS; Above shows the differences from our ideal table.

Transaction versus Session Key concept: When to delete the rows in the temporary table? Transaction level: delete rows in the temporary table when a COMMIT is issued Session level: delete rows when the user’s session ends (PRESERVE option) –EXIT in SQL*Plus We can use either one, depending upon how long we need the temporary data

Loading The Temporary Table CREATE GLOBAL TEMPORARY TABLE BB_WINNERS ( BB_SEQUENCE NUMBER(5) CONSTRAINT bb_seq_pk PRIMARY KEY, SCHOOL CHAR(30), YEAR NUMBER(4), TOURNAMENT CHAR(4), PLACE NUMBER(1), TIE CHAR(1)) ON COMMIT PRESERVE ROWS; INSERT INTO BB_WINNERS (SELECT BB_SEQ.NEXTVAL, NCAA_SCHOOL, NCAA_YEAR, ‘NCAA’, NCAA_PLACE, NULL FROM NCAA);

Adding In Our Other Table How to add the data from the NIT table: INSERT INTO BB_WINNERS (SELECT BB_SEQ.NEXTVAL, NIT_SCHOOL, NIT_YEAR, ‘NIT’, NIT_PLACE, NIT_TIE FROM NIT);

When To Use Temporary Tables? Performance may or may not suffer: –“The prudent use of temporary tables can dramatically improve Oracle SQL performance.” – Don Burleson –“Temporary tables slow performance dramatically.” – Justin Gunther (talking about Microsoft SQL Server) So…often a useful technique Use must be evaluated carefully in high- activity systems

Questions? For further information, refer to the CREATE statement in the SQL reference for the level of Oracle you are using.