SQL · Lesson 7 of 8
Creating tables & constraints
CREATE TABLE, data types, PRIMARY KEY, NOT NULL, UNIQUE, DEFAULT and FOREIGN KEY.
CREATE TABLE defines a table's columns and rules. Constraints protect your data:
PRIMARY KEY— unique id for each row.NOT NULL— the value is required.UNIQUE— no two rows may share the value (e.g. email).DEFAULT— value used when none is given.CHECK (salary_lpa > 0)— custom rule.REFERENCES other(id)— a foreign key pointing to another table.
Common types: INTEGER, REAL/DECIMAL, TEXT/VARCHAR(n), DATE, BOOLEAN (names vary slightly between databases).
Try it yourself
Press Run to see the results.
Runs on a small fictional sample database (companies, jobs) that is rebuilt on every run, so you can't break anything.
Key points
- Constraints stop bad data at the source.
- Foreign keys link tables.
- DEFAULT fills in missing values.
Quick check
Which constraint prevents two rows from having the same email?
UNIQUE rejects a value that already exists in that column.
Practice task
Try inserting a second applicant with the email 'anu@example.com' and read the error message.
Interview corner
What is normalization?
Organising tables to reduce duplication — for example storing company details once in companies and referencing them by id from jobs.
Primary key vs unique key?
A table has one primary key and it can't be NULL. It can have several UNIQUE constraints, and (in most databases) those columns may allow NULL.
Further reading
Official documentation, linked for deeper study. These are outside sites; this lesson's text is our own.
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