Database Modifications and Transactions

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1 Database Modifications and Transactions FCDB Dr. Chris Mayfield Department of Computer Science James Madison University Jan 31, 2018

2 pgadmin from home (the easy way) 1. Connect to JMU s network 2. Run web-based version of pgadmin Jan 31, 2018 Database Modifications and Transactions 2 of 17

3 pgadmin from home (the hard way) SSH tunnel from the command line: ssh -L 5432:data.cs.jmu.edu:5432 student.cs.jmu.edu Forward local traffic on port 5432 to data.cs.jmu.edu:5432 Then use localhost as the host in pgadmin on your machine On Windows/PuTTy: Connection > SSH > Tunnels Add new forwarded port: Source port 5432 Destination data.cs.jmu.edu:5432 Click Add; save profile, etc. Jan 31, 2018 Database Modifications and Transactions 3 of 17

4 Group Time Finalize names, create databases, get connected

5 Result tables PostgreSQL shortcut for saving query results CREATE TABLE table_name AS SELECT...; Today s goal: practice database updates In your group project database With multiple people updating! -- create a table of magic numbers CREATE TABLE magic AS SELECT i, i%5 as m, i^2 as s FROM generate_series(1, 20) AS g(i); -- change ownership to your group ALTER TABLE magic OWNER TO absent; Jan 31, 2018 Database Modifications and Transactions 5 of 17

6 Insertion Syntax INSERT INTO R(A 1,..., A n ) VALUES (v 1,..., v n ) Example INSERT INTO StarsIn(movieTitle, movieyear, starname) VALUES ('Maltese Falcon', 1942, 'Sydney Greenstreet'); Alternate INSERT INTO StarsIn -- without attribute list VALUES ('Maltese Falcon', 1942, 'Sydney Greenstreet'); What order should the values be in? Jan 31, 2018 Database Modifications and Transactions 6 of 17

7 INSERT Exercise Each group member should: (1) insert a unique row into the magic table (2) make sure you can see each other s rows

8 Bulk insertion Let s say we have: Studio(name, address, presc#) Movies(title, year, studioname, producerc#) For some reason, Studio doesn t have all studioname values! INSERT INTO Studio(name) SELECT DISTINCT studioname FROM Movies WHERE studioname NOT IN (SELECT name FROM Studio); What about address and presc# values? (see 6.5.1) How does the timing of insertions work? (see 6.5.2) Jan 31, 2018 Database Modifications and Transactions 8 of 17

9 Deletion Syntax DELETE FROM R WHERE <condition> Example DELETE FROM StarsIn WHERE movietitle = 'Maltese Falcon' AND movieyear = 1942 AND starname = 'Sydney Greenstreet'; Careful! DELETE FROM StarsIn WHERE networth < ; What if you forget the WHERE clause? Jan 31, 2018 Database Modifications and Transactions 9 of 17

10 DELETE Exercise Each group member should: (1) delete 2 original rows from the magic table (2) make sure you can see each other s changes

11 Updates Syntax UPDATE R SET <assignments> WHERE <condition> Example UPDATE MovieExec SET name = 'Pres. ' name WHERE cert# IN (SELECT presc# FROM Studio); Or better yet UPDATE MovieExec AS m SET name = 'Pres. ' name FROM Stuido AS s WHERE m.cert# = s.presc#; Remember: subqueries in WHERE are often join queries Jan 31, 2018 Database Modifications and Transactions 11 of 17

12 UPDATE Exercise Each group member should: (1) update their unique row from the magic table (2) make sure you can see each other s changes

13 And now, the fun part! Transactions in PostgreSQL

14 Transactions BEGIN; -- series of SQL statements COMMIT; Most databases run in auto-commit mode Until you execute BEGIN in psql/pgadmin If you want to undo everything BEGIN; -- series of SQL statements ROLLBACK; Jan 31, 2018 Database Modifications and Transactions 14 of 17

15 Transaction Exercise Each group member should: (1) BEGIN a transation (2) insert/update/delete magic rows (3) you shouldn t see each other s changes (4) COMMIT or ROLLBACK your transaction What happens if two people change the same row?

16 Multiversion Concurrency Control Internally, each transaction sees a snapshot of the database Version of the database when the transaction started -- system columns (x=transaction, c=command) SELECT xmin, cmin, xmax, cmax, * FROM magic -- or any table Advantages Locks for reading data do not conflict with locks for writing Improves concurrency performance and simplifies recovery Disadvantages Database can fill up with dead tuples over time DBMS must provide garbage collection (i.e., VACUUM) Jan 31, 2018 Database Modifications and Transactions 16 of 17

17 Modifying schemas Tables DROP TABLE Movies; DROP TABLE IF EXISTS Movies; -- postgres extension Columns ALTER TABLE MovieStar DROP birthdate; ALTER TABLE MovieStar ADD phone char(16); Default values In CREATE TABLE or ALTER TABLE statements: score integer DEFAULT 100 phone char(16) DEFAULT 'unlisted' birthdate date DEFAULT DATE ' ' Jan 31, 2018 Database Modifications and Transactions 17 of 17

Joins, NULL, and Aggregation

Joins, NULL, and Aggregation FCDB 6.3 6.4 Dr. Chris Mayfield Department of Computer Science James Madison University Jan 29, 2018 Announcements 1. Your proposal is due Friday in class Each group brings