A-level Computing/AQA/Paper 2/Fundamentals of databases/SELECT

Structured Query Language (SQL) UPDATE The SELECT statement allows you to ask the database a question (Query it), and specify what data it returns. We might want to ask something like Tell me the name and ages of all the crooks. Of course this wouldn't work, so we need to put this into a language that a computer can understand: Structured Query Language or SQL for short: SELECT name, DoB --what to return FROM crooks --where are you returning it from This would return the following: name DoB Geoff 12/05/1982 Jane 05/08/1956 Keith 07/02/1999 Oliver 22/08/1976 Kelly 11/11/1911 Marea 14/07/1940 But suppose we wanted to filter these results, for instance: Tell me the ID, name and ages of all the crooks who are male and come from Snape. We need to use another statement, the WHERE clause, allowing us to give the query some criteria (or options): SELECT ID, name, DoB FROM crooks WHERE town = 'Snape' AND gender = 'male' --Criteria This would return the following: ID name DoB 3 Keith 07/02/1999 Say the police knew that a crime had been committed by a heavily scarred woman (4+ scars), they want a list of all the scarred women: SELECT name, town, scars FROM crooks WHERE numScars >= 4 AND gender = 'female' --Criteria This would return: name town numScars Kelly East Ham 10 Marea Wythenshawe 6 However, the police want to quickly sort through and see who is the most heavily scarred. We are going to use an ORDER command: SELECT name, town FROM crooks WHERE numScars >= 4 AND gender = 'female' --Criteria ORDER BY numScars DESC --sorts the numScars values in big to small order ORDER BY numScars sorts your returned data into DESCending (big to small) or ASCending (small to big) order Extension:   WARNING: in the exam they will likely ask you to return specific fields so using a * (wildcard character) will lose you marks! Sometimes you would like to return all the details regarding a record, this is a little cumbersome if you have to list them: SELECT name, DoB, gender, town .... Instead we can use the *. This means return all fields from a selected record: SELECT * --return all fields from crooks FROM crooks WHERE name = 'Jane' This would return: id name gender DoB town numScars 2 Jane female 05/08/1956 York 1 1 Exercise: SELECT statements Using the criminals data set, write SQL to find the name and location of women born before 1993 SELECT name, town FROM crooks WHERE DoB < 1993 AND gender = 'female' Using the above data set, write a Query to find out the town and gender for all persons called Noel: SELECT town, gender FROM crooks --you don't have to have a separate line for each SQL statement WHERE name = 'Noel' The police are pretty sure that a criminal is either a woman OR a person from Newcastle. Find their names and DoB SELECT name, DoB FROM crooks WHERE gender = 'female' OR town = 'Newcastle' You can build even more complex SQL WHERE statements by combining ANDs and ORs. Maybe the police should have said that a criminal is either a woman, OR a man AND from Newcastle. SELECT name, DoB FROM crooks WHERE gender = 'female' OR (gender = 'male' AND town = 'Newcastle') Using the above data set, write a Query to find the ages of crooks from youngest to oldest SELECT name, town, DoB FROM crooks ORDER BY DoB DESC INNER JOIN We spoke earlier about how databases are great because they allow you to link tables together and perform complex searches on linked data. So far we have only looked at searching one table. When you use a social network such as Facebook you can often see a list of all your friends in the side bar as well as details from your record such as your name and place of work. How did they find this data? They would have searched for all the relationships that involve your ID, returning the names of people involved AND returned values such as job title from your personal record. This looks like using two queries: --return relationship information—return personal record information It would be possible to do this, but it's far easier to use one query for both things. Take a look at this example. The police want to know the name and town of a criminal (ID = 45) along with all the descriptions of crimes they have performed: SELECT name, town, description --select things to return (from different tables) FROM crooks, crime --name tables that data comes from WHERE crook.Id = crime.crimId --specify the link dot.notation means table.field. The Ids are the same AND crook.Id = 45 --specify which crook you are looking at ORDER BY date ASC --order the results by the oldest first Operators used in the WHERE clauses Operator Meaning of the operator Example = Checks if they're equivalent Id1 = 123 > and < Checks if a field is greater than or less than Id1 > 123 Id1 < 123 <> Checks if it's not equal to Id1 <> 123 >= and <= Similar to "> and <", but also checks for equality Id1 >= 123 Id1 <= 123 OR Will be accepted if either the left, right or both are true Id1 = 123 OR Id2 <> 444 AND Only accepted if both the left part and the right part are true Id1 = 123 AND Id2 <> 444 NOT Inverts the boolean value of the statement NOT Id1 = 123 IS NULL Checks if it's a null value contained within the variable Id1 IS NULL ... BETWEEN ... AND ... Checks if something is within a range Id1 BETWEEN 2.8 AND 3.14159265