Sql Basic Selects

SQL Select
. . . the very basics

Bob Litsinger
bob.litsinger@gmail.com

Introduction
 This slide set covers basic select
statements against a single table.
 It does not cover:
 Joins and other table to table interactions.
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 Using sub-queries in a primary query.
 The use of the many functions available in SQL.
 There are a few examples of functions, and
an additional topics list at the end.

Contents
SQL Select . . . the very basics
Introduction 2 More on ORDER BY 13
Contents 3 Having 14
English like 4 Operators 15
Simple Select 5 Qualifiers for WHERE 16
Specifying the fields 6 More on qualifiers for WHERE 17
Introducing WHERE clauses 7 BETWEEN with datetime type 18
Bob Litsinger
bob.litsinger@gmail.com Exploring information 8 Wildcards with LIKE 19
Count – How Many Products 9 ALIASES, CAST and CONVERT 20
A few aggregations 10 Other Capabilities 21
Establishing order 11 Queries Unleashed 22
More on GROUP BY 12

English like
 Basic SQL is very intuitive for users
because a request is English like:

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 . . . but using this structure, we can
ask complicated questions.

Simple Select
 I wonder what types of products
Adventure * Will show us all the
details (called fields) about
the Categories

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 To much information? You can get
just the ID and Name by being more
specific.

Specify the fields
 So, let’s get the Category ID, Name
and Manager:

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 Using the Category ID for Bikes
(1), the types of bikes can be found
in the Subcategory table.

Introducing WHERE clauses
 To limit the Sub Categories to bikes, a
WHERE clause is added:

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Exploring information
 Looking only at Road Bikes in the
Product line up will enable us to
explore some data in other ways.

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 Gives us a list of 43 bikes

More are listed

Count - How Many Products
 The count(*) function provides a
simple count of the bikes
This assigns a name to a column. If the
name has spaces, keywords or some
other characters it requires brackets.

Bob Litsinger All Products

The word AS is optional, but makes
things clearer. If there are no spaces or
invalid characters, brackets are not
needed.

Just Road Bikes

Introducing column labels

A few aggregations.
Aggregations allow I can add a label to each row. It is listed as a query
item and is in quotes.
analysis of numeric values.
Simple count of number of
Road 52 cm road bikes by color
Finds the minimum, i.e.
lowest, value

Calculates the average value.

Finds the maximum, i.e.
Bob Litsinger highest, value
To calculate the Notice the WHERE clause can
aggregates the include many conditions
query needs to
know how the
listing is grouped.

A variation: Finds the distinct number of colors of road bikes
(for all sizes).

Establishing order
SUM() is the most frequently This is a function that returns the number of the
month. Functions will be discussed briefly later.
used aggregate.

Notice something else new.
An ORDER BY clause was
added to put the months in
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More on GROUP BY

If the GROUP BY
clause is left off

If the GROUP BY
clause is incomplete
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Color is not listed!

However, if the aggregate
applies only to the entire
query with no breakdown
by any category, then a
GROUP BY is not needed.

More on ORDER BY
 It is always the last cause.
Sorting is the last step for the
SQL engine, and the request
for it is placed last.

 Can be against multiple
fields, and any field order.
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 Does not need to agree with
the fields or field order in
GROUP BY

 Is the only place in a query
script where a field alias can
be used.

HAVING
Works like a “WHERE” clause but for aggregations

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Operators
+ Addition and Concatenation
Used in WHERE clause . . .
- Subtraction
* Multiplication
/ Division
= Equals
<> Does Not Equal
!= Does Not Equal
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bob.litsinger@gmail.com >= Greater Than or Equal To
<= Less Than or Equal To
> Greater Than
< Less Than
% Mod operator (also used as wildcard)

. . . or to calculate a value.

Qualifiers for WHERE
There are some special operators that can be used in the WHERE
clause.
These can be used with NOT.

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For LIKE there are even some
special wildcard characters.

More on qualifiers for
WHERE

IN and NOT IN

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Be sure your range is “inclusive” when you use between.

Using a single beginning letter
will not get the Subcategories
that begin with “M.” To get all
these add zzz:

With datetime data
type

BETWEEN with datetime type
The datetime data type has a time with it. If the data actually records
time between will leave out records if you rely on implicit text to
datetime conversion. This small test tables shows the problem.

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These two queries solve the problem:

Both get all the records.

Wildcards with LIKE

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Aliases, Convert and Cast
 Some ways to make things more readable are
shown in the slide
 Many of the slides use field aliases to provide user
friendly column labels.
 CAST and CONVERT are both used in places. These make
query results more presentable.
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 There are other options (like STR) and
CONVERT/CAST setting that you can use. There
are specific settings in CONVERT for date formats.

Other Capabilities
 Advanced aggregations like CUBE, ROLLUP and
PIVOT
 Specific Math, String, Date, Text, Image and other
Functions

Bob Litsinger  Program controls like IF, WHILE, CASE, etc.

 Stored Procedures and Functions
 System Functions and Procedures
 Triggers and Database Management

Queries Unleashed
To really tapped the power of a relational
database by using multiple tables.

 Use tables together with JOIN, UNION, EXCEPT
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and MERGE.

 Use sub-queries to help the extend the power of a
primary query.

Check out other existing and future slide shows
for examples of other SQL functions, capabilities
and scripting.

Sql Basic Selects

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Sql Basic Selects