Transcript Databases Tutorial 1

Databases Week 1, lab 2
Simple selects
About the environment
• We are using SQL Server for the moment.
• The server we are using is:
– Cian.student.comp.dit.ie
• The method of authentication you are
using is SQL Server authentication.
• Your username and password is:
– DT2112<your username>
• E.g. mine would be DT2112pobyrne
About the environment
• When you connect, you are automatically
directed to the database of which you are
the owner.
– Please note: SQL Server refers to your
collection of tables as your database. Oracle
refers to this area as your schema.
• There are other databases to which you
have read access. This means you can
run SELECT statements against them.
Other databases
• There are two Microsoft supplied sample
databases.
– These are PUB and NORTHWIND.
• I have also set up a database that contains
several models.
– It is called DT2112examples.
• You have read access to all of these databases.
• You do not have read access to the models of
anyone else in your class, or in another class.
Exercises
• Today, you will be doing very simple exercises
on the database.
• Connect to your own database, as you did on
Tuesday, using the Query Analyser.
• Switch to whichever database is mentioned in
the exercise, either by entering USE
NORTHWIND (for example) or by using the
drop-down box at the top of your screen.
• Try the examples and exercises in blue.
Simple selects
• The full syntax of the SELECT statement is
complex, but the main clauses can be
summarized as:
• SELECT select_list
[INTO new_table_name]
FROM table_list
[WHERE search_conditions]
[GROUP BY group_by_list]
[HAVING search_conditions]
[ORDER BY order_list [ASC | DESC] ]
• In this lesson, we will address only those
clauses in black.
Select list
• Describes the columns of the result set. It is a
comma-separated list of expressions.
• Each expression defines both
– the format (data type and size) and
– the source of the data for the result set column.
• Each select list expression is usually a
reference to a column in the source table or view
the data is coming from, but can be any other
expression, such as a constant or a TransactSQL function.
• Using the * expression in a select list specifies
that all columns in the source table are returned.
FROM table_list
• Contains a list of the tables from which the
result set data is retrieved. These sources
can be:
– Base tables in the local server running
Microsoft® SQL Server™.
– Views in the local SQL Server. SQL Server
internally resolves a view reference to
references against the base tables that make
up the view. See later in the course.
Simplest select
• Select * from <table-name>
– E.g. Select * from dog
• Try to do this example, using the
DT2112examples database.
Selection
• Selection picks individual columns from a
table:
• Try it!
Miscellaneous
• To comment out parts of a query, surround
the part by /*…*/
• To make the analyser pick up after an
error or break, use the word GO
• Write a little query with comments on the
top, with a select and a GO.
• Try again, but this time do 2 queries, with
a deliberate error in the first one.
To put a name on a column
• Transact SQL allows the use of the ‘as’ clause to give a
name to a column.
– Unitprice as Price or
– UnitPrice as ‘Unit Price’
• This can be used on any column, but is especially useful
in a derived column.
• New columns can be derived from existing fields:
• E.g. the value of an item in stock is the number in stock
by the unit price.
• Write a query to select the companyname field from
NORTHWIND’s shippers table, calling it Shipper.
SQL
• SQL stands for Structured Query Language.
There is an ANSI Standard 2003 for SQL.
• Most relational database vendors hit the
standards in spots, but don’t stick rigorously to
them.
• The functions are often a place where the
standards are ignored.
• Some of the functions used for SQL Server will
be quite different when you use Oracle.
Cast and CONVERT
• CAST and CONVERT
• Explicitly converts an expression of one
data type to another. CAST and
CONVERT provide similar functionality.
• This can be used to format output and
convert data fields from one format to
another
Cast and Convert
• CAST and CONVERT
– Explicitly converts an expression of one data type
to another. CAST and CONVERT provide similar
functionality.
• Syntax
–
–
–
–
Using CAST:
CAST ( expression AS data_type )
Using CONVERT:
CONVERT ( data_type [ ( length ) ] , expression [ ,
style ] )
Number conversion
• When data types are converted with a different number
of decimal places, the value is truncated to the most
precise digit.
– For example, the result of SELECT CAST(10.6496 AS int) is 10.
• When data types in which the target data type has fewer
decimal points than the source data type are converted,
the value is rounded.
– For example, the result of CAST(10.3496847 AS decimal(6,2))
is 10.35.
• Write a query to return the product name and price (with
2 decimal places) of each product in the NORTHWIND
products table.
B. Use CAST with arithmetic
operators
• This example calculates a single column
computation (Copies) by dividing the total yearto-date sales (ytd_sales) by the individual book
price (price).
• This result is converted to an int data type after
being rounded to the nearest whole number.
USE pubs
GO
SELECT CAST(ROUND(ytd_sales/price, 0) AS int) AS 'Copies'
FROM titles
GO
• Try it!
C. Use CAST to concatenate
• This example concatenates
noncharacter, nonbinary expressions
using the CAST data type conversion
function.
– USE pubs
– GO
– SELECT 'The price is ' + CAST(price AS
varchar(12)) FROM titles WHERE price >
10.00
– GO
D. Use CAST for more readable
text
• This example uses CAST in the select
list to convert the title column to a
char(50) column so the results are
more readable.
– USE pubs
– GO
– SELECT CAST(title AS char(50)), ytd_sales
FROM titles
– GO
Syntax of CAST
• Using CAST:
• CAST ( expression AS data_type )
• This allows us to do the following:
– Format the output so that it is more
readable.
– Change a number to a character string.
– Change a character string to a numeric
format.
Using CAST
Exercises
• Retrieve the system date, using the
‘GetDate’ function.
• Convert it into char(12) format.
• Display it using style 3.
• Now display it using styles 1 through 14,
noting the differences.
• Which of these formats do you think would
be most useful in this country and for what
purposes?