SQL

A database table stores records in rows and fields in columns. SQL uses SELECT to choose fields to show, FROM to name the table, and WHERE to keep only rows matching a condition. For the students table below, SELECT name, score FROM students WHERE year = 10 returns only the Year 10 students' names and scores:

SELECT * would show every field. The WHERE condition filters records; it does not change the stored table. SQL keywords are commonly written in capitals, but they are not case-sensitive.

A WHERE condition can compare a field with a value using =, !=, <, <=, > or >=. AND and OR join two conditions:

• SELECT name FROM students WHERE year = 10 AND score > 75 returns only Ada: she is in Year 10 and scored over 75. Cy is in Year 10 but scored 74.
• SELECT name FROM students WHERE year = 11 OR score > 80 returns Ada (score 81) and Ben (Year 11).

With AND, a record must meet both conditions. With OR, it must meet at least one.

To work out what a query returns, read it in this order:

1. FROM: find the table.
2. WHERE: go down the table one record at a time, keeping only those that meet the condition.
3. SELECT: from the records kept, show only the fields listed, in the order they are listed.

Then write the result as a small table, with the field names as its headings. If no record meets the condition, the query returns no records: that's a valid answer, not an error.