SQL Homework Help

About SQL

This hub covers databases and the language used to query them: what a database is, how tables relate, keys and normalisation, and the SQL statements that create, read and change data. The questions run from definitions and normal forms to writing a SELECT for a given requirement.

Popular topics in SQL

• Relational Databases

  Tables, records and fields, the relational model, and the advantages over flat files.

• Keys

  Primary, composite, foreign and candidate keys, referential integrity and how relationships are enforced.

• SQL Queries

  SELECT with WHERE, ORDER BY, aggregate functions, GROUP BY and HAVING.

• Joins

  Inner and outer joins, joining on a foreign key, and querying across several tables.

• Data Definition & Manipulation

  CREATE, ALTER and DROP, and INSERT, UPDATE and DELETE with their constraints.

• Normalisation

  First, second and third normal form, partial and transitive dependency, and why redundancy is removed.

• Constraints

  NOT NULL, UNIQUE, CHECK and DEFAULT, and validation at the database level.

• Database Design

  Entity-relationship diagrams, cardinality, and turning a scenario into a schema.

How ScanSolve helps with SQL

ScanSolve writes the query and then explains each clause, so the answer shows why the WHERE and the GROUP BY are where they are. For design questions the schema is given as tables with their keys marked, which is the form these answers are marked in.

Normalisation questions are answered stage by stage — the dependency that breaks the current form, then the tables that result — rather than asserting the final schema. Where a question describes a requirement in words, the answer restates it as the conditions the query must express before writing any SQL.

Study tips for SQL

1. 01

   Learn the clause order: SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY. Queries fail on order more than on syntax.

2. 02

   Identify the primary key of each table before drawing relationships. Everything else in design follows from the keys.

3. 03

   For normalisation, name the dependency you are removing at each step. That is the mark, not the final tables.

4. 04

   Remember that WHERE filters rows and HAVING filters groups. Questions about aggregates test precisely that.

Frequently asked SQL questions

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