COMP102: Introduction to Databases, 20

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1 COMP102: Introduction to Databases, 20 Dr Muhammad Sulaiman Khan Department of Computer Science University of Liverpool U.K. 29 March, 2011

2 SQL, part 7

3 Specific topics for today: More about constraints Triggers

4 Table constraints When creating a table we can add constraints to it: PRIMARY KEY: Defines a column as the primary key, as previously discussed. FOREIGN KEY: Defines a column as a foreign key, as previously discussed. CONSTRAINT name CHECK (expression): The name is just an identifier for that particular constraint. The expression for the check must evaluate to a true or false value. This can be used, for example to ensure that one value is greater than another.

5 CHECK constraints The expression is very similar to the WHERE clause of a query. AND, OR and NOT for boolean operators Mathematical functions and operators Comparisons (=, <>), etc. Table and columns Specified values New or updated rows must fulfill the condition for the operation to succeed.

6 Example: CHECK constraints We don t want to allow the price of an item to be negative, but there isn t a default type for nonnegativeinteger: CREATE TABLE products ( identifier integer, price numeric, name varchar, CONSTRAINT priceokay CHECK (price > 0) );

7 Example: CHECK constraints We can also have CHECK constraints for a column rather than the table: CREATE TABLE products ( identifier INT, price NUMERIC CHECK (price > 0) ); Or: CREATE TABLE products ( identifier INT, price NUMERIC CONSTRAINT priceokay CHECK (price > 0) );

8 Example: CHECK constraints We need to use a table constraint to check more than one column. We want the time a video is due back to the store to always be after when it was rented: CREATE TABLE rentals ( identifier INT, timeout TIMESTAMP, dueback TIMESTAMP, CONSTRAINT inoutokay CHECK (timeout < dueback) );

9 Example: CHECK constraints We can use other comparison operators, like IN and LIKE: CREATE TABLE Weblog ( identifier INT, webpage VARCHAR, day VARCHAR(3), CONSTRAINT webpageokay CHECK (webpage LIKE ), CONSTRAINT dayokay CHECK (day IN ( MON, TUE,...)) );

10 Altering constraints on tables Refering to the previous slide we can drop constraints by: ALTER TABLE Weblog DROP CONSTRAINT webpageokay; We can also add new constraints by: ALTER TABLE Weblog ADD CONSTRAINT indentifieriskey PRIMARY KEY (indentifier); ALTER TABLE Weblog ADD CONSTRAINT webpageokay2 CHECK (webpage LIKE OR webpage LIKE );

11 CHECK constraints Why? We could always check in our program that the value is correct. There may be many programs inserting data into the database. Those programs might be written by people other than the administrator of the database. Overall: It promotes data integrity and consistency.

12 Triggers Trigger is a series of actions that are associated with certain events, such as insertions, deletions or updates to a particular table, and that are performed whenever these events arise. Triggers are also called event-condition-action (ECA) rules. Differently from CHECK constraints, triggers do not immediately prevent the event but they first test a condition. If the condition does not hold, then the trigger performs no action in connection with a given event. If the condition of the trigger is satisfied, then the action associated with the trigger is performed. The action (any sequence of database operations) may then prevent the event from taking place, or it could undo the event (e.g., delete the row inserted).

13 Triggers in SQL The action may be executed either before or after the triggering event. The action can refer to both old and/or new values of records that were inserted, deleted or updated in the event that triggered the action. Update events may be limited to a particular attribute or set of attributes. A condition may be specified by a WHEN clause; the action is excuted only if the trigger is triggered and the condition holds when the triggering event occurs. The programmer has an option of specifying that the action is performed either: once for each modified record, or once for all the records that are changed in one database operation.

14 Example: Trigger-1 Consider the following schema: MovieProducer(name, address, certno, networth) We ll define a trigger triggered by updates to the networth to prevent any attempt to lower the net worth of a movie producer. (1) CREATE TRIGGER NetWorthTrigger (2) AFTER UPDATE OF networth ON MovieProducer (3) REFERENCING (4) OLD ROW AS OldTuple, (5) NEW ROW AS NewTuple (6) FOR EACH ROW (7) WHEN (OldTuple.netWorth > NewTuple.netWorth) (8) UPDATE MovieProducer (9) SET networth = OldTuple.netWorth (10) WHERE certno = NewTuple.certNo; Line (2): triggering event; Lines (3)-(5): set up the objects the condition and action will operate on; Line (6): means that this trigger is executed once for each updated record; Line (7): condition part; Lines (8)-(10): action part (here, an ordinary SQL statement).

15 Triggers general issues Line (2) in previous example says that trigger is executed after the triggering event (AFTER). Replacing AFTER by BEFORE causes the WHEN condition to be tested before the event. If the condition is true, then the trigger s action is executed; and then the event is executed, regardless of whether the condition is true. Triggering events: UPDATE, INSERT, DELETE: The OF networth clause is optional for UPDATE and if present defines the event to be only update of attributes listed after OF. Clause OF not permitted for INSERT and DELETE events; these events make sense for entire records only. The WHEN clause is optional. If missing, the action is executed whenever the trigger is triggered. Previous example shows a single SQL statement as an action. There can be any number of such statements separated by semicolons and surrounded by BEGIN... END.

16 Triggers general issues When the triggering event is UPDATE then old (before update) and new (after update) records are referred to by using OLD ROW AS and NEW ROW AS. When the triggering event is INSERT then we may use NEW ROW AS to give name to the inserted record, and OLD ROW AS is disallowed. When the triggering event is DELETE then we may use OLD ROW AS to give name to the deleted record, and NEW ROW AS is disallowed.

17 Triggers general issues Row-level trigger: statement FOR EACH ROW (line (6) in previous example) present. Statement-level trigger: FOR EACH ROW omitted (and default FOR EACH STATEMENT present). A statement-level trigger is executed once whenever an event statement (such as one in line (2) in previous example; not the action statement!) of the appropriate type is executed, no matter how many (0 or more) rows it affects. Example: If the entire table updated with an SQL update statement, a statement-level trigger executes only once, while a row-based trigger executes once for each record to which an update is applied. Any trigger (of any kind) can refer to the table of old records (deleted records or old version of updated ones) and to the table of new records (inserted records or new versions of updated records), using declarations OLD TABLE AS OldStuff and NEW TABLE AS NewStuff.

18 Example: Trigger-2 Consider the same schema: MovieProducer(name, address, certno, networth) We ll define a trigger to prevent the average net worth of all movie producers from dropping below $500, 000. This constraint can be violated by an insertion, deletion or update to networth column. Note: The average net worth may drop below $500, 000 during an (event) statement which may execute many modifications (e.g., single UPDATE statement may change many records of table MovieProducer). We only want to prevent it dropping below $500, 000 at the end of the statement. It is necessary to write one trigger for each of these events of insertion, deletion and update of table MovieProducer. We will only define a trigger for the update event (the other triggers are similar and in fact simpler).

19 Example: Trigger-2, contd (1) CREATE TRIGGER AvgNetWorthTrigger (2) AFTER UPDATE OF networth ON MovieProducer (3) REFERENCING (4) OLD TABLE AS OldStuff, (5) NEW TABLE AS NewStuff (6) FOR EACH STATEMENT (7) WHEN ( > ( SELECT AVG(netWorth) FROM MovieProducer ) ) (8) BEGIN (9) DELETE FROM MovieProducer (10) WHERE (name, address, certno, networth) IN NewStuff; (11) INSERT INTO MovieProducer (12) (SELECT * FROM OldStuff); (13) END; Lines (3)-(5): declare that NewStuff and OldStuff are the names of tables containing (only) the new records and old records involved in the database operation that awakened the trigger. Note: One SQL statement can modify many records, and thus there can be many records in NewStuff and OldStuff. Line (6): means that this trigger is executed once for an event statement (here update) regardless of how many records it affected. Line (7): condition that is true if the average net worth is below $500, 000 after the update statement. Lines (8)-(13): action restores the old table MovieProducer if the condition of WHEN clause is satisfied.