The SELECT statement is probably the most commonly used in SQL. It simply retrieves data from the database.
Lets have a look at a simple SELECT statement:
SELECT * FROM Individual
This SQL SELECT statement is attempting to retrieve all columns from a table called Individual.
How do we know it is trying to select all columns? Because it is using an asterisk (*). This is a quick way of selecting all columns – it’s much easier than writing out the names of all columns (especially if there are a lot of columns).
Of course, this SQL SELECT statement assumes that there is a table called Individual. If there wasn’t, an error would be generated.
Lets have a look at the table the statement is trying to select data from:
Because our select statement asks to display all columns and all records, we would see the following:
Select from Multiple Tables
You can select from more than one table at a time. To do this, simply separate each table with a comma. You should also qualify any references to columns by placing the table name in front, separated by a dot.
We have another table called Occupation, which contains the individual’s occupation.
We will select from both the Individual table and the Occupation table. We will qualify any column names by prefixing them with its table’s name and a dot.
SELECT * FROM Individual, Occupation WHERE Individual.FirstName = 'Homer'
Displaying Less Columns
If you don’t need every column to be displayed you can single out just the columns you’re interested in. It’s good programming practice to do this – the more columns your program has to return, the more it will impact its performance.
To only display those columns you’re interested in, simply replace the asterisk (*) with a comma separated list of the column names.
SELECT IndividualId, LastName, UserNameFROM Individual WHERE FirstName = 'Homer'