DML: SELECT

CIS-182

SELECT Queries

• Used to display data and calculations• Main tool for making use of the data that’s

stored in tables– SELECT can be used as part of other

commands

Key Words

• SELECT {field list}: What’s returned• FROM {data source}: Which table to use• WHERE {criteria}: Which rows to include• GROUP BY {field list}: How to summarize• HAVING {field list}: Which groups to

include when grouping• ORDER BY {field list}: How to sort

How Process

1. FROM – get data from original source(s)2. WHERE – limit rows to work with3. GROUP BY – create groups4. HAVING – limit groups5. SELECT – specify what to return6. ORDER BY – specify sorting

SELECT

• SELECT starts statement• Followed by field list– Fields can refer to table fields or calculations

created as part of query

• SELECT can be used by itself:SELECT GetDate()

FROM

• Describes where to find the data• Can refer to a table or view – View is a pre-defined select query, considered

a “virtual” table

• Basic query typically has SELECT and FROM

SELECT * FROM Titles

Using SELECT

• INTO – write results into table• DISTINCT – eliminate repeating value(s) or

rows• AS – name or rename a table or column• * - include all fields from source

INTO

• Optional clause can write data from one or more tables into another (new) table

• Typically used for – Archiving– To speed processing by creating temporary or

summary tables– To organize for reporting

SELECT pub_state INTO PublisherStates FROM Publishers

DISTINCT

• Distinct eliminates repeating rows or values– Used in SELECT clause

SELECT DISTINCT Title FROM TitlesEnsures that a title only is returned once, regardless of how may times a title is found

SELECT DISTINCT pubid FROM TitlesLists each publisher in the titles table just once, regardless of number of books published

AS

• AS is used to create a field name for a calculation or to rename an existing field – Also referred to as an alias– Use is optional

Rename fields in authors:SELECT au_lname AS LastName FROM AuthorsSELECT au_fname FirstName FROM Authors

*

• * represents all fields in data source– Results may differ from what’s expected if

fields are added, deleted or order changed

SELECT * FROM Publishers

Where

• WHERE limits which rows are included in the result

• Criteria evaluate to True/False– True means row will be used for result, false

means row will be excluded– Sometimes ‘Predicates’ is used in place of

criteria (a predicate is a characteristic that’s true about a row)

Comparison Operators

• Relational (=, >, <,<>,>=,<=)• Like – compares text patterns– Wildcards

• One Character: _ (underscore)• Any combination: %

• Between – compares to range• Is – compares true/false/null• In – value is in a list• Exists – whether there’s a value

Sample WHERE Clauses

SELECT *FROM TitlesWHERE Price > 20

SELECT au_LnameFROM AuthorsWHERE State <> ‘wa’

Logical Operators

• If have more than one test use AND/OR to tie tests together– AND: All parts must evaluate to True for a row

to be included– OR: Any part evaluating to True means a row is

included– Precedence: NOT then AND then OR

• NOT returns inverse– Not False returns True

Logical Operator Examples

SELECT * FROM Titles WHERE Price > 20 AND Title LIKE ‘%SQL%’

SELECT *FROM TitlesWHERE Not Contract

Group By

• Allows data from tables to be summarized– Find rows that share a common value

• Can discover information about the group using aggregate functions– Count– Average– Max

Sample Grouping

• Count number of books by publisher:SELECT Count(*) BookCount, pub_idFROM titlesGROUP BY pub_id

• Count books by priceSELECT Price, Count(*) AS BookCountFROM TitlesGROUP BY Price

Having

• Determines which groups to include• Use Criteria similar to Where clause– Test characteristics of group using HAVING– Use WHERE to determine which rows to include

Sample Having

• Display publisher ID’s of those publishers having more than 5 books published:SELECT pub_idFROM TitlesGROUP BY pub_idHAVING Count(*) > 5

Order By

• Provides means to sort results by a column (actual or calculated)

• Default sort order is Ascending

ORDER BY pub_dateORDER BY au_lname DESC