SQL continued CMSC 461 Michael Wilson. Right back into it Recap: Learned how to log in to...

SQL continuedCMSC 461Michael 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 <column1, column2, column3> FROM <table_name1>, <table_name2>WHERE <conditions>

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 <column> <op>

<column/constant/formula> <op> 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 exampleaddress daysSinceConta

ctcontactPhoneNumber

numberType contactName

111 Great Street

1 555 5555 Cell Phil

8 Get Out of Here Way

7 555 7777 Work Bob

7 RUN! Drive 700 555 8888 Cell Octavio21 Jump Street

40 123 4567 Cell Johnny

Complete select example SELECT *

FROM AddressBookWHERE daysSinceLastContact > 20

Complete select example SELECT contactName

FROM AddressBookWHERE phoneType = ‘Cell’




111 Great Street




Complete select example SELECT contactName

FROM AddressBookWHERE phoneType = ‘Cell’ and contactName = ‘Octavio’




7 RUN! Drive 700 555 8888 Cell Octavio

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 <table>, <table>SET <column> = <value>, <column> = <value> …WHERE <where-clause>

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 exampleaddress daysSinceConta



111 Great Street



7 555 7777 Work Bob



Update example UPDATE AddressBook

SET phoneNumber = ‘444 7777’WHERE contactName = ‘Phil’




111 Great Street



7 555 7777 Work Bob



Update example UPDATE AddressBook

SET daysSinceLastContact = 0, phoneType = ‘Home’WHERE contactName = ‘Octavio’




111 Great Street



7 555 7777 Work Bob

7 RUN! Drive 0 555 8888 Home Octavio21 Jump Street


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