1. Basic Data Manipulation - Reading Data
Objectives
To learn how to carry out simple manipulation on the data being read
Contents
The SELECT Statement
The SELECT and FROM clauses
Limiting columns, calculated columns, built-in functions, aliases
The DISTINCT keyword
Sorting the result set - the ORDER BY clause
Practical 3-1
Row Selection options - the WHERE clause
Practical 3-2
Nulls, nullability, three way logic
Practical 3-3

2. SELECT * FROM company Simple SELECT SQL Command Verb Columns to
Display
SELECT *
FROM company
Table
to Select Data
from

3. SELECT * -- all columns FROM salesperson Statement Format
SQL is a free format language, but syntactically extremely fussy
use new lines, tab keys and indentation to make it readable
white space is ignored by the parser
Make use of comments
SELECT * all columns
FROM salesperson

4. SELECT company_no, name, county FROM company
Specifying Columns
You have 2 choices
an ‘*’, or else list the columns comma-separated
if there are 40 columns and you want only 39 of them, then list them
columns may be listed in any order
Columns to
Display
SELECT company_no, name, county FROM company

5. Calculated (Virtual) Columns and Aliases
Some RDBMS support this 2nd notation for aliases
Real
Column
Calculated (virtual)
Column
SELECT lname,
sales_target * 1.2 AS ‘next year’
FROM salesperson
Column
Alias
SELECT lname, ‘next year’ = sales_target * 1.2
FROM salesperson

6. Standard and non-Standard Functions
SQL2 provides a standard set of scalar functions
implementations vary - different names and arguments
most engines offer further, non-standard functions
A SQL standard function:
SELECT order_no, Substring (their_order_no,1,3 )
FROM sale
Non-standard examples
SELECT order_no, Datepart(qq, order_date ) qtr_sold
FROM sale (MS SQL Server)
SELECT order_no, TO_CHAR(order_date,’Q’ ) qtr_sold
FROM sale (Oracle)

7. Keyword - Distinct emp_no dept_no sales_target 10 1 23000 20 3 34500
‘DISTINCT’ (if used) always follows SELECT
emp_no dept_no sales_target
salesperson
Result
SELECT DISTINCT dept_no, ….
FROM salesperson
dept_no 1 2 3

8. Sorting the Results By Specific Column(s) SELECT * FROM contact
NOTE - an ‘alias’ (defined in the SELECT clause) can only be used elsewhere in ‘ORDER BY’
Each part of the sort sequence is ‘ASC’ending unless DESC specified
SELECT *
FROM contact
ORDER BY company_no DESC, name
By Alias Name
… ORDER BY month

9. Ch9 Practical1 - Basic Selects (single table)
Limiting columns
Calculations using functions
Aliases
Sorting and removing duplicates

10. Limiting rows with basic operators
Think of a WHERE clause as an ‘IF’ statement
A Basic Operator
SELECT *
FROM sale
WHERE order_value > 2000
Value
Limits rows to those
matching the condition
Numeric column

11. BETWEEN SELECT * FROM salesperson
Saves typing “col >= x AND col <= y” but means the same thing.
SELECT *
FROM salesperson
WHERE sales_target BETWEEN 100 AND 500
Starting Value
Stopping Value

12. IN Peter George Tom Mike Sandy Eleanor Bill Gary Grace Harry Samantha
Dick
SELECT fname
FROM salesperson
WHERE fname IN (‘Fred’, ‘Tom’, ‘Dick’, ‘Harry’)
-- easier than coding
WHERE fname = ‘Fred’ OR
fname = ‘Tom’ OR
fname = ‘Dick’ OR
fname = ‘Harry’
Tom
Dick
Harry
salesperson
output

13. NOT Peter George Tom Mike Sandy Eleanor Bill Gary Grace Harry Samantha
Dick
SELECT fname
FROM salesperson
WHERE fname NOT IN (‘Fred’, ‘Tom’, ‘Dick’, ‘Harry’)
Peter
George
Mike
Sandy
Eleanor
Gary
Grace
Samantha
salesperson
output

14. LIKE WHERE lname LIKE ‘A%’ - (starts with ‘A’)
….. lname LIKE ‘%A’ - (ends with ‘A’)
….. lname LIKE ‘%A%’ - (contains an ‘A’)
WHERE lname LIKE ‘_T%N%R_’
means - has a ‘T’ in position 2
and has an ‘R’ in the penultimate position
and has an ‘N’ somewhere in between
eg ‘stationary’ would match
‘_’ Matches any single character
‘%’ Matches any number of characters (incl 0!)
Note – LIKE may only be used with character columns

15. Multiple Conditions SELECT * FROM company WHERE county = ‘Surrey’
When mixing AND’s and OR’s use parentheses to clarify meaning
SELECT *
FROM company
WHERE county = ‘Surrey’
AND town = ‘Harlow’
First condition
Second condition
Connector
CONNECTOR TYPE PRECEDENCE
AND Evaluated first
OR Evaluated second

16. Ch9 Practical2 - Basic Selects (SingleTable)
Restricting rows using a ‘WHERE’ clause

17. Nulls SELECT * FROM salesperson WHERE notes IS NULL
Basic premise of an RDBMS is the concept of optional columns
NULL means ’not applicable’ or ’unknown’
NULL is different from zero or blank
On INSERT of a new row, must supply values for mandatory columns
other columns will be NULL (assuming no ‘DEFAULT’ value)
NULL propagates through expressions: null = null, not 5
An expression in a WHERE clause may evaluate to TRUE, FALSE or NULL
only rows whose expressions evaluate to TRUE are output
Use IS NULL to retrieve rows with NULL entries:
SELECT *
FROM salesperson
WHERE notes IS NULL

18. Ch9 Practical3 - Basic Selects (SingleTable)
Using NULLS and functions relating to NULLS

19. SUMMARY SELECT retrieves data
The SELECT ‘columnlist’ says which columns
FROM says which table, WHERE says which rows
ORDER BY sorts, DISTINCT ensures no two rows of the output are the same
NULLS are an unfortunate necessity and will come up many times
You can type the word SELECT without thinking
But you can’t type column names after it without having worked out the FROM clause!
Remember this when you try to READ SQL later!!
The logical sequence is actually
FROM this table
From the rows WHERE this is true
Show (SELECT) me these columns or calculations