SQL introduction

RHS – SOC 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

RHS – SOC 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…?

RHS – SOC 4 Getting data out of databases NameAddressDate of birthOccupationIncome John None$20,000 Mira Doctor$120,000 Alex Programmer$240,000 Steven Teacher$60,000 Joanne Doctor$90,000 Hannah Nurse$45,000 Susanne None$20,000

RHS – SOC 5 Getting data out of databases NameAddressDate of birthOccupationIncome John None$20,000 Mira Doctor$120,000 Alex Programmer$240,000 Steven Teacher$60,000 Joanne Doctor$90,000 Hannah Nurse$45,000 Susanne None$20,000 ”Older than 40 years”

RHS – SOC 6 Getting data out of databases NameAddressDate of birthOccupationIncome Mira Doctor$120,000 Steven Teacher$60,000 Joanne Doctor$90,000 Susanne None$20,000

RHS – SOC 7 Getting data out of databases NameAddressDate of birthOccupationIncome Mira Doctor$120,000 Steven Teacher$60,000 Joanne Doctor$90,000 Susanne None$20,000 ”Income of more than $50,000”

RHS – SOC 8 Getting data out of databases NameAddressDate of birthOccupationIncome Mira Doctor$120,000 Steven Teacher$60,000 Joanne Doctor$90,000

RHS – SOC 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

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

RHS – SOC 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!

RHS – SOC 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

RHS – SOC 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

RHS – SOC 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

RHS – SOC 15 Exercise The piece of Java code to retrieve records was potentially very inefficient, since all records might be examined in the process 1.Consider how the data in the recordList could be organised, in order to make it more efficient to find all records for persons older than 40 2.Will the improved structure also make the original query more efficient (persons older than 40 and income more than $50,000)?