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SQLintroduction
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
Name Address Date of birth Occupation Income
John --- 12-05-1980 None $20,000
Mira --- 30-01-1956 Doctor $120,000
Alex --- 09-10-1971 Programmer $240,000
Steven --- 19-06-1966 Teacher $60,000
Joanne --- 03-12-1961 Doctor $90,000
Hannah --- 22-03-1977 Nurse $45,000
Susanne --- 01-08-1960 None $20,000
RHS – SOC 5
Getting data out of databases
Name Address Date of birth Occupation Income
John --- 12-05-1980 None $20,000
Mira --- 30-01-1956 Doctor $120,000
Alex --- 09-10-1971 Programmer $240,000
Steven --- 19-06-1966 Teacher $60,000
Joanne --- 03-12-1961 Doctor $90,000
Hannah --- 22-03-1977 Nurse $45,000
Susanne --- 01-08-1960 None $20,000
”Older than 40 years”
RHS – SOC 6
Getting data out of databases
Name Address Date of birth Occupation Income
Mira --- 30-01-1956 Doctor $120,000
Steven --- 19-06-1966 Teacher $60,000
Joanne --- 03-12-1961 Doctor $90,000
Susanne --- 01-08-1960 None $20,000
RHS – SOC 7
Getting data out of databases
Name Address Date of birth Occupation Income
Mira --- 30-01-1956 Doctor $120,000
Steven --- 19-06-1966 Teacher $60,000
Joanne --- 03-12-1961 Doctor $90,000
Susanne --- 01-08-1960 None $20,000
”Income of more than $50,000”
RHS – SOC 8
Getting data out of databases
Name Address Date of birth Occupation Income
Mira --- 30-01-1956 Doctor $120,000
Steven --- 19-06-1966 Teacher $60,000
Joanne --- 03-12-1961 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)?
