SQL Queries — A-Level Computer Science Revision
Revise SQL Queries for A-Level Computer Science. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel and OCR.
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Go to NormalisationWhat is SQL Queries?
SQL (Structured Query Language) is a programming language that is used to manage and query relational databases. It can be used to create, modify, and delete tables, as well as to insert, update, and delete data. It can also be used to retrieve data from a database using queries.
Board notes: AQA, Edexcel, and OCR all require students to be able to write SQL queries to retrieve data from a relational database.
Step-by-step explanationWorked example
To retrieve the names of all students who are enrolled in the computer science course, you would use the following SQL query: `SELECT name FROM students WHERE student_id IN (SELECT student_id FROM enrollments WHERE course_id = 'CS101');` This query first selects the student IDs of all students who are enrolled in the computer science course, and then it selects the names of those students from the students table.
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Common mistakes
- 1Incorrectly using the syntax of SQL queries.
- 2Not understanding the different types of joins.
- 3Writing inefficient queries that take a long time to run.
SQL Queries exam questions
Exam-style questions for SQL Queries with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for SQL Queries
Core concept
SQL (Structured Query Language) is a programming language that is used to manage and query relational databases. It can be used to create, modify, and delete tables, as well as to insert, update, and …
Frequently asked questions
What is the difference between an inner join and an outer join?
An inner join returns only the rows that have matching values in both tables. An outer join returns all of the rows from one table and the matching rows from the other table. If there is no match, the result is NULL on the side of the table that does not have a match.
What is an aggregate function?
An aggregate function is a function that performs a calculation on a set of values and returns a single value. Examples of aggregate functions include COUNT, SUM, AVG, MIN, and MAX.