1. Dept. of Computing Science, University of Aberdeen1 Writing SELECT SQL Queries Nigel Beacham n.beacham@abdn.ac.uk www.abdn.ac.uk/~edu196 based on materials by Dr Yaji Sripada

2. Dept. of Computing Science, University of Aberdeen2 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

3. Dept. of Computing Science, University of Aberdeen3 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

4. Dept. of Computing Science, University of Aberdeen4 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

5. Dept. of Computing Science, University of Aberdeen5 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 FROM A,B ;

6. Dept. of Computing Science, University of Aberdeen6 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)

7. Dept. of Computing Science, University of Aberdeen7 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,Z FROM A,B

8. Dept. of Computing Science, University of Aberdeen8 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 like SELECT X,Y,Z*2 AS ZDoubled FROM A,B

9. Dept. of Computing Science, University of Aberdeen9 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 like SELECT MAX(X),MIN(Y),AVG(Z) FROM A,B All columns are aggregated and size of the results table is 1 row

10. Dept. of Computing Science, University of Aberdeen10 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 like SELECT X,MIN(Y),AVG(Z) FROM A,B GROUP BY X 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

11. Dept. of Computing Science, University of Aberdeen11 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 like SELECT X, MIN(Y), AVG(Z) FROM A,B GROUP BY X HAVING 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

12. Dept. of Computing Science, University of Aberdeen12 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

13. Dept. of Computing Science, University of Aberdeen13 Step3 CASE1: –Non-join conditions are usually written based on the constraints mentioned in the query specification –At this stage your query may look like SELECT X, Y, Z FROM A,B WHERE X = AND Y = ;

14. Dept. of Computing Science, University of Aberdeen14 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 like SELECT X,Y,Z FROM A,B WHERE X = AND Y = (SELECT COUNT(*) FROM C) ;

15. Dept. of Computing Science, University of Aberdeen15 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, Z FROM A,B WHERE A. =B. AND X = AND Y = ;

16. Dept. of Computing Science, University of Aberdeen16 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.

17. Dept. of Computing Science, University of Aberdeen17 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.