Presentation on theme: "Database Systems SQL Queries, PL/SQL, Triggers Gergely Czuczy Cause Light Wounds: I call upon chaos to cause unbalanced parentheses."— Presentation transcript:
Database Systems SQL Queries, PL/SQL, Triggers Gergely Czuczy Cause Light Wounds: I call upon chaos to cause unbalanced parentheses.
Todays Database FIRST, run this script: _AFTER_ the first, run this one:
(NOT) EXISTS You can satisfy queries where you have to select things which are related to all other things. For an example select those pilots who know all the planes.
(NOT) EXISTS First, select all the planes(q1). From q1 subtract the planes that the actual pilot knows(q2) You will get of a set of planes from q2. We have to select those pilots where this set is empty.
(NOT) EXISTS SELECT p1.name FROM pilots p1 WHERE NOT EXISTS ( (SELECT id FROM planes) MINUS (SELECT k1.planeid FROM knows k1 WHERE k1.pilotid = p1.id) ) ;
PL/SQL With PL/SQL you will be able to do many jobs in oracle that require programming, such as writing triggers, functions, and so on. In a PL/SQL block you can use queries such in anywhere else.
PL/SQL The PL/SQL block: DECLARE BEGIN END
PL/SQL Declaring variables: Right after the DECLARE keyword ; Example: emp_name varchar2(1024); current date; nextid integer;
PL/SQL IF-statements: IF THEN ELSIF THEN END IF; The ELSIF branch is optional
Trigger A trigger executes a procedure for some kind of action. For an example on every INSERT statement on a table a trigger executed that logs the newly inserted row. Triggers can be fired before or after some kind of action
Trigger + PL/SQL Triggers are mostly written in PL/SQL Special variables that you can use: –NEW: the newly inserted row’s data, or the updated data in an UPDATE query –OLD: the old data in a DELETE/UPDATE trigger Where refering to NEW/OLD you have to prefix it by a ‘:’, ex: :NEW.personid
Trigger Example #1 CREATE OR REPLACE TRIGGER trg_knows BEFORE DELETE OR INSERT ON knows FOR EACH ROW DECLARE BEGIN IF INSERTING THEN INSERT INTO logs (systime,action) VALUES (sysdate,:NEW.pilotid||' has learned plane '||:NEW.planeid); ELSIF DELETING THEN INSERT INTO logs (systime,action) VALUES (sysdate,:NEW.pilotid||' has forgotten plane '||:NEW.planeid); END IF; END;
Trigger Example #2 CREATE OR REPLACE TRIGGER trg_knows_update BEFORE UPDATE ON knows FOR EACH ROW DECLARE BEGIN :NEW.pilotid := :OLD.pilotid; :NEW.planeid := :OLD.planeid; END;
Sequence A sequence stores a value that can be incremented by a function call. It’s useful for automagicaly creating ID’s. To create a sequence use: CREATE SEQUENCE some_seq; To get the next value use: some_seq.nextval();