Dept. of Computing Science, University of Aberdeen 1

Writing SELECT SQL Queries

Nigel Beachamn.beacham@abdn.ac.uk

www.abdn.ac.uk/~edu196based on materials by Dr Yaji

Sripada

Dept. of Computing Science, University of Aberdeen 2

Disclaimer• Writing SELECT queries is a skill you need a lot in

your future courses and also in your jobs.• Following some simple steps/guidelines in writing

these queries will help you to develop this skill in a systematic way.

• These notes contain steps/guidelines for writing some types of SELECT queries– Queries that are known to be difficult for learners

• These are informal notes and students should use them only in the initial stages of learning SQL until they develop their own procedure for writing SQL queries.

• Please feel free to modify them – they are ‘copyleft’ http://www.gnu.org/copyleft/#WhatIsCopyleft

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SELECT Queries

• A SELECT query retrieves information from a database and structures them into a results table

• There are many types of SELECT queries as described in Ch6 C&B.

• But we consider the following types– Using aggregate functions

• Without Group By• With Group By

– Subqueries– Joins

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Write SELECT Queries in 3 steps

• Step 1: Decide the tables you require for answering the query

• Step 2: Decide the columns you require in your results table

• Step 3: Decide the rows you require in your results table

Dept. of Computing Science, University of Aberdeen 5

Step1

• Decide the tables you require for answering the query– Based on the information you want to show to

the user, you select the tables which contain this information.

– At this stage you have information to write the FROM clause of your SELECT query

– Your query may look like (A and B are Tables)SELECT <columns to be decided>FROM A,B<everything else to be decided>;

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Step2

• Decide the columns you require in your results table– Normally this is quite simple

• CASE1: The query specification contains a list of required columns.

– But in some cases this may be involved• CASE2: If you need a calculated field in your results

table (such as deposit calculated from monthly rent)• CASE3: If you need aggregated information in your

results table (such as count of staff, average salary etc)

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Step2

• CASE1:– You simply list the required columns in

the SELECT clause– At this stage your query may look like

(X,Yand Z are columns in A or B)SELECT X,Y,ZFROM A,B<everything else to be decided>

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Step2

• CASE2:– Initially list all the simple required

columns– In addition, compute the required

calculated columns and set their names (headings) using AS

– At this stage your query may look likeSELECT X,Y,Z*2 AS ZDoubledFROM A,B<everything else to be decided>

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Step2

• CASE3:– Compute the required aggregated columns and

set their names using AS– Because you used aggregate functions in the

column list YOU ARE NOT ALLOWED TO USE ANY OTHER UNAGREGATED COLUMNS IN THE COLUMN LIST

– At this stage your query may look likeSELECT MAX(X),MIN(Y),AVG(Z)FROM A,B<everything else still to be decided>

All columns are aggregated and

size of the results table is

1 row

Dept. of Computing Science, University of Aberdeen 10

Step2

• CASE3 continued:– You can show unaggregated columns in

the SELECT clause only if you group their items using the GROUP BY clause

– At this stage your query may look likeSELECT X,MIN(Y),AVG(Z)FROM A,BGROUP BY X<everything else still to be decided>

Items in X are grouped, therefore X can be included in the column list.

Size of the results table is equal to the number of groups of items

Dept. of Computing Science, University of Aberdeen 11

Step2

• CASE3 continued:– Sometimes you want to control the

groups of items you show in the result table using HAVING condition

– At this stage your query may look likeSELECT X, MIN(Y), AVG(Z)FROM A,BGROUP BY XHAVING <Some-Condition><Everything else still to be decided>

HAVING is used only to restrict the groups shown in the result table. WHERE is

used to restrict the rows shown in the result table

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Step3• Decide the rows you require in your results

table– This involves composing several conditional

expressions each of which constrains the rows shown in the results table in some way

– Join all these conditional expressions using logical operators such as AND and OR in the WHERE clause

– There are three different kinds of conditional expressions you write

• CASE1: Conditional expressions specified in the query specification (such as show details for ‘Aline Stewart’)

• CASE2: Conditional expressions based on results from other query

• CASE3: Conditional expressions you require for joining tables

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Step3

• CASE1:– Non-join conditions are usually written

based on the constraints mentioned in the query specification

– At this stage your query may look likeSELECT X, Y, ZFROM A,BWHERE X = <Some Value>AND Y = <Some Value><join conditions still to be specified>;

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Step3• CASE2:

– Sometime non-join conditions involve checking values not specified by the user (As in CASE1) but present elsewhere in the database – same table or other tables

– You need to first write a SELECT query to obtain this information

• You follow the same steps recommended here to write the subquery

– Once the subquery is ready you then use it as part of the conditional expression

– At this stage your query may look likeSELECT X,Y,ZFROM A,BWHERE X = <Some-Value>AND Y = (SELECT COUNT(*) FROM C)<join conditions still to be specified>;

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Step3

• CASE3:– Join conditions are decided based on the type of join you

wish to perform (refer to the different types of joins from the lecture notes)

– The most common type of join is a natural join or inner join

– For every pair of tables you join, determine the links (foreign key links) between them

– Compose a join condition for each of the links– At this stage your query is complete and may look like

SELECT X, Y, ZFROM A,BWHERE A.<PrimaryKey>=B.<ForeignKey>AND X = <Some Value>AND Y = <Some Value>;

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Verification Tips

• After you write a query always try to ‘hand run’ it to see if it retrieves the required information

• For this purpose, it is a good idea you imagine you are marking a query written by some other person to avoid the obvious conflict of interest between you as the author and you as the marker.

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Appeal

• The cases covered in these steps are by no means complete

• You can add many more cases to these steps to include other types of SQL queries from Ch5 C&B.

• If you manage to write additional cases into the ‘3 step process’ described here please let me know - I might use them in the future.

• If you find any errors (of any kind) in these notes please let me know.

• I am happy to acknowledge your efforts on these notes.