SQL introduction. SWC - 2011 2 Getting data out of databases Databases are ”just” containers of...
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Transcript of SQL introduction. SWC - 2011 2 Getting data out of databases Databases are ”just” containers of...
SQLintroduction
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
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…?
SWC - 2011 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
SWC - 2011 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”
SWC - 2011 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
SWC - 2011 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”
SWC - 2011 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
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
SWC - 2011 10
Getting data out of databases
for (Records r : recordList)
{
if ((r.getAge() > 40) and
(r.getIncome() > 50000))
{
selectedRecords.add();
}
}
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!
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
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
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
