CE 279 - WRITING SQL QUERIES umat edition.ppt

SQL
Structured Query Language
1

SQL is an ANSI (American National Standards Institute)
standard computer language for accessing and
manipulating database systems. SQL statements are used
to retrieve and update data in a database. SQL works with
database programs like MS Access, DB2, Informix, MS SQL
Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the
SQL language, but to be in compliance with the ANSI
standard, they must support the same major keywords in
a similar manner (such as SELECT, UPDATE, DELETE,
INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their
own proprietary extensions in addition to the SQL
standard!
SQL is a Standard - BUT....
2

SQL Database Tables
A database most often contains one or more tables. Each table is
identified by a name (e.g. "Customers" or "Orders"). Tables contain
records (rows) with data.
Below is an example of a table called "Persons":
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
3

With SQL, we can query a database and have a result set returned.
A query like this:
SELECT LastName FROM Persons
Gives a result set like this:
SQL Queries
LastName
Hansen
Svendson
Pettersen
4

SQL Data Manipulation
Language (DML)
SQL (Structured Query Language) is a syntax for executing
queries. But the SQL language also includes a syntax to
update, insert, and delete records.
These query and update commands together form the Data
Manipulation Language (DML) part of SQL:
•SELECT - extracts data from a database table
•UPDATE - updates data in a database table
•DELETE - deletes data from a database table
•INSERT INTO - inserts new data into a database table
5

SQL Data Definition Language
(DDL)
The Data Definition Language (DDL) part of SQL permits
database tables to be created or deleted. We can also define
indexes (keys), specify links between tables, and impose
constraints between database tables.
The most important DDL statements in SQL are:
•CREATE TABLE - creates a new database table
•ALTER TABLE - alters (changes) a database table
•DROP TABLE - deletes a database table
•CREATE INDEX - creates an index (search key)
•DROP INDEX - deletes an index
6

SQL The SELECT Statement
The SELECT statement is used to select data from a table. The
tabular result is stored in a result table (called the result-set).
Syntax
SELECT column_name(s)
FROM table_name
7

To select the columns named "LastName" and "FirstName",
use a SELECT statement like this:
SELECT LastName, FirstName FROM Persons
Persons
výsledok
LastName FirstName
Hansen Ola
Svendson Tove
Pettersen Kari
8

To select all columns from the "Persons" table, use a *
symbol instead of column names, like this:
SELECT * FROM Persons
Select All Columns
9

The result from a SQL query is stored in a result-set. Most database
software systems allow navigation of the result set with
programming functions, like: Move-To-First-Record, Get-Record-
Content, Move-To-Next-Record, etc.
Programming functions like these are not a part of this tutorial. To
learn about accessing data with function calls, please visit our ADO
tutorial.
The Result Set
10

Semicolon is the standard way to separate each SQL statement in
database systems that allow more than one SQL statement to be
executed in the same call to the server.
Some SQL tutorials end each SQL statement with a semicolon. Is
this necessary? We are using MS Access and SQL Server 2000
and we do not have to put a semicolon after each SQL statement,
but some database programs force you to use it.
Semicolon after SQL Statements?
11

The DISTINCT keyword is used to return only distinct (different)
values.
The SELECT statement returns information from table columns.
But what if we only want to select distinct elements?
With SQL, all we need to do is to add a DISTINCT keyword to the
SELECT statement:
Syntax
SELECT DISTINCT column_name(s)
FROM table_name
The SELECT DISTINCT Statement
12

To select ALL values from the column named "Company" we use
a SELECT statement like this:
SELECT Company FROM Orders
Orders
Company OrderNumber
Sega 3412
W3Schools 2312
Trio 4678
W3Schools 6798
Company
Sega
W3Schools
Trio
W3Schools
Using the DISTINCT keyword
13

Note that "W3Schools" is listed twice in the result-set.
To select only DIFFERENT values from the column named
"Company" we use a SELECT DISTINCT statement like this:
SELECT DISTINCT Company FROM Orders
Company
Sega
W3Schools
Trio
Orders
Company OrderNumber
Sega 3412
W3Schools 2312
Trio 4678
W3Schools 6798
14

Select All Columns
The WHERE clause is used to specify a selection criterion.
The WHERE Clause
To conditionally select data from a table, a WHERE clause can be
added to the SELECT statement.
Syntax
SELECT column FROM table
WHERE column operator value
15

With the WHERE clause, the following operators can be
used:
Operator Description
= Equal
<> Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN Between an inclusive range
LIKE Search for a pattern
Note: In some versions of SQL the <> operator may be written as !=
16

Using the WHERE Clause
To select only the persons living in the city "Sandnes", we add a
WHERE clause to the SELECT statement:
WHERE City='Sandnes'
LastName FirstName Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
Pettersen Kari Storgt 20 Stavanger 1960
LastName FirstName Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
17

Using Quotes
Note that we have used single quotes around the conditional
values in the examples.
SQL uses single quotes around text values (most database
systems will also accept double quotes). Numeric values should
not be enclosed in quotes.
For text values:
This is correct:
SELECT * FROM Persons WHERE FirstName='Tove'
This is wrong:
SELECT * FROM Persons WHERE FirstName=Tove
18

The LIKE Condition
The LIKE condition is used to specify a search for a pattern in
a column.
Syntax
SELECT column FROM table
WHERE column LIKE pattern
A "%" sign can be used to define wildcards (missing letters
in the pattern) both before and after the pattern.
19

Using LIKE
The following SQL statement will return persons with first names
that start with an 'O':
WHERE FirstName LIKE 'O%'
that end with an 'a':
WHERE FirstName LIKE '%a'
20

Using LIKE 2
that contain the pattern 'la':
WHERE FirstName LIKE '%la%'
21

SQL The INSERT INTO
Statement
22

The INSERT INTO Statement
The INSERT INTO statement is used to insert new rows into
a table.
Syntax
INSERT INTO table_name
VALUES (value1, value2,....)
You can also specify the columns for which you want to insert
data:
INSERT INTO table_name (column1, column2,...)
VALUES (value1, value2,....)
23

Insert a New Row
And this SQL statement:
INSERT INTO Persons
VALUES ('Hetland', 'Camilla', 'Hagabakka 24', 'Sandnes')
Hetland Camilla Hagabakka 24 Sandnes
24

Insert Data in Specified Columns
And This SQL statement:
INSERT INTO Persons (LastName, Address)
VALUES ('Rasmussen', 'Storgt 67')
Rasmussen Storgt 67
25

The Update Statement
The UPDATE statement is used to modify the data in a table.
Syntax
UPDATE table_name
SET column_name = new_value
WHERE column_name = some_value
27

Update one Column in a Row
Nilsen Fred Kirkegt 56 Stavanger
Rasmussen Storgt 67
We want to add a first name to the person with a last name of
"Rasmussen":
UPDATE Persons SET FirstName = 'Nina'
WHERE LastName = 'Rasmussen'
Rasmussen Nina Storgt 67
28

Update several Columns in a
Row
We want to change the address and add the name of the city:
UPDATE Persons
SET Address = 'Stien 12', City = 'Stavanger'
WHERE LastName = 'Rasmussen'
Rasmussen Nina Stien 12 Stavanger
Rasmussen Storgt 67
29

The Delete Statement
The DELETE statement is used to delete rows in a table.
Syntax
DELETE FROM table_name
WHERE column_name = some_value
31

32

Delete a Row
"Nina Rasmussen" is going to be deleted:
DELETE FROM Persons WHERE LastName = 'Rasmussen'
33

Delete All Rows
It is possible to delete all rows in a table without deleting the
table. This means that the table structure, attributes, and indexes
will be intact:
DELETE FROM table_name
Or
DELETE * FROM table_name
34

CE 279 - WRITING SQL QUERIES umat edition.ppt

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