Fundamentals of databases: SELECT

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PAPER 2 - ⇑ Fundamentals of databases ⇑

← Structured Query Language (SQL) SELECT UPDATE →

The SELECT statement allows you to ask the database a question (Query it), and specify what data it returns. We might want to ask something like Tell me the name and ages of all the crooks. Of course this wouldn't work, so we need to put this into a language that a computer can understand: Structured Query Language or SQL for short:

SELECT name, DoB --what to return
FROM crooks      --where are you returning it from

This would return the following:

name DoB
Geoff 12/05/1982
Jane 05/08/1956
Keith 07/02/1999
Oliver 22/08/1976
Kelly 11/11/1911
Marea 14/07/1940

But suppose we wanted to filter these results, for instance: Tell me the ID, name and ages of all the crooks who are male and come from Snape. We need to use another statement, the WHERE clause, allowing us to give the query some criteria (or options):

SELECT ID, name, DoB 
FROM crooks      
WHERE town = 'Snape' AND gender = 'male'  --Criteria

This would return the following:

ID name DoB
3 Keith 07/02/1999

Say the police knew that a crime had been committed by a heavily scarred woman (4+ scars), they want a list of all the scarred women:

SELECT name, town, scars
FROM crooks      
WHERE numScars >= 4 AND gender = 'female'  --Criteria

This would return:

name town numScars
Kelly East Ham 10
Marea Wythenshawe 6

However, the police want to quickly sort through and see who is the most heavily scarred. We are going to use an ORDER command:

SELECT name, town 
FROM crooks      
WHERE numScars >= 4 AND gender = 'female'  --Criteria
ORDER BY numScars DESC --sorts the numScars values in big to small order

ORDER BY numScars sorts your returned data into DESCending (big to small) or ASCending (small to big) order

Exercise: SELECT statements

Using the criminals data set, write SQL to find the name and location of women born before 1993


SELECT name, town 
FROM crooks      
WHERE DoB < 1993 AND gender = 'female'

Using the above data set, write a Query to find out the town and gender for all persons called Noel:


SELECT town, gender FROM crooks  --you don't have to have a separate line for each SQL statement
WHERE name = 'Noel'

The police are pretty sure that a criminal is either a woman OR a person from Newcastle. Find their names and DoB


SELECT name, DoB FROM crooks
WHERE gender = 'female' OR town = 'Newcastle'

You can build even more complex SQL WHERE statements by combining ANDs and ORs. Maybe the police should have said that a criminal is either a woman, OR a man AND from Newcastle.

SELECT name, DoB FROM crooks
WHERE gender = 'female' OR (gender = 'male' AND town = 'Newcastle')

Using the above data set, write a Query to find the ages of crooks from youngest to oldest


SELECT name, town, DoB FROM crooks

INNER JOIN[edit | edit source]

We spoke earlier about how databases are great because they allow you to link tables together and perform complex searches on linked data. So far we have only looked at searching one table.

When you use a social network such as Facebook you can often see a list of all your friends in the side bar as well as details from your record such as your name and place of work. How did they find this data? They would have searched for all the relationships that involve your ID, returning the names of people involved AND returned values such as job title from your personal record. This looks like using two queries: --return relationship information—return personal record information It would be possible to do this, but it's far easier to use one query for both things.

Take a look at this example. The police want to know the name and town of a criminal (ID = 45) along with all the descriptions of crimes they have performed:

SELECT name, town, description  --select things to return (from different tables)
FROM crooks, crime              --name tables that data comes from
WHERE crook.Id = crime.crimId   --specify the link dot.notation means table.field.  The Ids are the same
  AND crook.Id = 45             --specify which crook you are looking at
ORDER BY date ASC               --order the results by the oldest first

Operators used in the WHERE clauses[edit | edit source]

Operator Meaning of the operator Example
= Checks if they're equivalent Id1 = 123
> and < Checks if a field is greater than or less than
  • Id1 > 123
  • Id1 < 123
<> Checks if it's not equal to Id1 <> 123
>= and <= Similar to "> and <", but also checks for equality
  • Id1 >= 123
  • Id1 <= 123
OR Will be accepted if either the left, right or both are true Id1 = 123 OR Id2 <> 444
AND Only accepted if both the left part and the right part are true Id1 = 123 AND Id2 <> 444
NOT Inverts the boolean value of the statement NOT Id1 = 123
IS NULL Checks if it's a null value contained within the variable Id1 IS NULL
... BETWEEN ... AND ... Checks if something is within a range Id1 BETWEEN 2.8 AND 3.14159265