1. SQL introduction

2. SWC - 2011 2 Getting data out of databases Databases are ”just” containers of data We could – in principle – just put data in a text file instead The real strength of databases is the ability to efficiently retrieve a specified subset of data

3. SWC - 2011 3 Getting data out of databases Suppose we have information about a set of persons: –Name –Address –Date of birth –Occupation –Income –… We wish to find all persons, who are older than 40 years, and have an income of more than $50,000 Is that an easy or hard task…?

4. SWC - 2011 4 Getting data out of databases NameAddressDate of birthOccupationIncome John---12-05-1980None$20,000 Mira---30-01-1956Doctor$120,000 Alex---09-10-1971Programmer$240,000 Steven---19-06-1966Teacher$60,000 Joanne---03-12-1961Doctor$90,000 Hannah---22-03-1977Nurse$45,000 Susanne---01-08-1960None$20,000

5. SWC - 2011 5 Getting data out of databases NameAddressDate of birthOccupationIncome John---12-05-1980None$20,000 Mira---30-01-1956Doctor$120,000 Alex---09-10-1971Programmer$240,000 Steven---19-06-1966Teacher$60,000 Joanne---03-12-1961Doctor$90,000 Hannah---22-03-1977Nurse$45,000 Susanne---01-08-1960None$20,000 ”Older than 40 years”

6. SWC - 2011 6 Getting data out of databases NameAddressDate of birthOccupationIncome Mira---30-01-1956Doctor$120,000 Steven---19-06-1966Teacher$60,000 Joanne---03-12-1961Doctor$90,000 Susanne---01-08-1960None$20,000

7. SWC - 2011 7 Getting data out of databases NameAddressDate of birthOccupationIncome Mira---30-01-1956Doctor$120,000 Steven---19-06-1966Teacher$60,000 Joanne---03-12-1961Doctor$90,000 Susanne---01-08-1960None$20,000 ”Income of more than $50,000”

8. SWC - 2011 8 Getting data out of databases NameAddressDate of birthOccupationIncome Mira---30-01-1956Doctor$120,000 Steven---19-06-1966Teacher$60,000 Joanne---03-12-1961Doctor$90,000

9. SWC - 2011 9 Getting data out of databases Pretty easy; we could almost do it just by looking at the table What if we had 100,000 records…? Maybe I could write a small program to pick out the relevant records

10. SWC - 2011 10 Getting data out of databases for (Records r : recordList) { if ((r.getAge() > 40) and (r.getIncome() > 50000)) { selectedRecords.add(); }

11. SWC - 2011 11 Getting data out of databases Fine, but… The code is very task-specific; can only solve this specific problem The code might be inefficient; we examine all records to find the specified subset Enter SQL!

12. SWC - 2011 12 SQL SQL – Structured Query Language A language which enables us to specify subsets of data in a database Subsets in terms of –Tables –Fields –Conditions on fields

13. SWC - 2011 13 SQL SQL is the standard for database languages It is non-procedural; you specify what data to find, not how to find it Fairly easy to learn and understand

14. SWC - 2011 14 SQL SQL enables us to –Create databases and tables –Perform ”data management” like inserting, updating and deleting records –Perform queries on data; i.e. retrieve specified subsets of data We concentrate on queries initially