Be able to use SQL to retrieve data from a relational database, using the…

The idea

SQL is the language used to query a relational database. To retrieve data you write a SELECT statement naming the columns you want, then FROM the table that holds them. A WHERE clause filters rows by a condition, for example WHERE YearGroup = 10, using the equality operator to match a value. Comparison operators such as > and < test ranges, and AND or OR combine conditions. Text values go in single quotation marks; numbers do not. The result is a set of rows containing only the requested columns that satisfy the condition. Practise reading a small table, predicting which rows a given statement returns, and spotting errors such as a missing FROM or a misspelt column name.

What earns the marks

• SELECT names the columns to return; FROM names the table they come from.
• WHERE filters rows using a condition, for example WHERE YearGroup = 10, with the equality operator between column and value.
• Comparison operators such as >, < and <> test values; AND and OR combine two or more conditions.
• Text values are enclosed in single quotation marks, while numeric values are written without quotes.
• The output contains only the selected columns and only the rows that satisfy the WHERE condition.

Common mistakes and how to put them right

• Writing the equality test with a word instead of the symbol.

  Put it right: Place the equals sign between the column name and the value, as in WHERE Score = 50.

• Omitting the FROM clause.

  Put it right: To state the table after FROM so the database knows where to look.

• Putting numbers in quotation marks or leaving text unquoted.

  Put it right: Quote text values only and write numeric values bare.

Exam tips

• Trace a statement clause by clause: SELECT, then FROM, then WHERE, checking each column name against the table.
• When asked what a query returns, list the columns and the qualifying rows rather than describing the statement in general terms.
• Check quotation marks and operators carefully, because a single wrong symbol changes which rows are returned.