SQL continued CMSC 461 Michael Wilson

Right back into it  Recap:  Learned how to log in to PostgreSQL  Learned about PostgreSQL data types  Learned how to create tables and insert data into tables

Select statement  This is how you get data out of tables  This is the great majority of most applications SQL calls  Basic syntax:  SELECT FROM, WHERE

Select statement  Selecting columns  This tells what column values you want coming back in your results  Can be any subset of the columns available in a table  If you want all columns, you can type * instead of listing out the columns  SELECT * FROM test_table WHERE…

Select statement  Selecting tables  You can select from any number of tables  Let’s focus on the use case of just one for now  We will address multi-table selects later

Select statement  Where clause  This is exactly like the conditions piece of the relational algebra select  You are stringing together specific conditions that you want the returned data to meet

Where clauses  Where clauses consist of a series of predicates  These predicates are the terms discussed during the relational algebra lecture  The format is the same   is =, !=, >, >=, <, <=  Can also use <> instead of !=

Predicate examples  address = ‘21 Jump Street’  age >= 21  bloodType = ‘O+’

Where clauses  You can string predicates together using AND or OR

Stringing together predicates  age > 21 AND bloodType = ‘O+’  daysSinceContact > 50 or myOpinionOfThisPerson = ‘Favorable’

Complete select example address daysSinceContact contactPhoneNumber numberTypecontactName 111 Great Street CellPhil 8 Get Out of Here Way WorkBob 7 RUN! Drive CellOctavio 21 Jump Street CellJohnny

Complete select example  SELECT * FROM AddressBook WHERE daysSinceLastContact > 20

Complete select example address daysSinceContact contactPhoneNumber numberTypecontactName 7 RUN! Drive CellOctavio 21 Jump Street CellJohnny

Complete select example  SELECT contactName FROM AddressBook WHERE phoneType = ‘Cell’

Complete select example address daysSinceContact contactPhoneNumber numberTypecontactName 111 Great Street CellPhil 7 RUN! Drive CellOctavio 21 Jump Street CellJohnny

Complete select example  SELECT contactName FROM AddressBook WHERE phoneType = ‘Cell’ and contactName = ‘Octavio’

Complete select example address daysSinceContact contactPhoneNumber numberTypecontactName 7 RUN! Drive CellOctavio

Predicate formulas  You can also use formulas in predicates  Example  age < (20 + 5)  Why this is useful will make more sense later, however

Update statement  This will update specific values in already existing rows in your table  Basic syntax:  UPDATE, SET =, = … WHERE

Update statement  Table selection  You can select multiple tables during one update  Let’s focus on the single table case for now

Update statement  Column value setting  Column = new value  The new value can be another column, a value, or a formula (similar to how predicates work in where clauses)  Can set multiple columns in one statement

Update statement  Where clause  This where clause functions exactly the same as it does with a select statement

Update example address daysSinceContact contactPhoneNumber numberTypecontactName 111 Great Street CellPhil 8 Get Out of Here Way WorkBob 7 RUN! Drive CellOctavio 21 Jump Street CellJohnny

Update example  UPDATE AddressBook SET phoneNumber = ‘ ’ WHERE contactName = ‘Phil’

Update example address daysSinceContact contactPhoneNumber numberTypecontactName 111 Great Street CellPhil 8 Get Out of Here Way WorkBob 7 RUN! Drive CellOctavio 21 Jump Street CellJohnny

Update example  UPDATE AddressBook SET daysSinceLastContact = 0, phoneType = ‘Home’ WHERE contactName = ‘Octavio’

Update example address daysSinceContact contactPhoneNumber numberTypecontactName 111 Great Street CellPhil 8 Get Out of Here Way WorkBob 7 RUN! Drive HomeOctavio 21 Jump Street CellJohnny