SQL Constraint: Main Tips
- An SQL constraint defines a rule to the data records in tables.
- When an infraction against the data record action and the constraint occurs that action is stopped by it.
- They can be defined after or when creating the table.
Constraint Syntax and Quick Reference
CREATE TABLE table_name ( column1 datatype constraint_one, column2 datatype constraint_two, column3 datatype constraint_three, .... );
The main constraints in SQL are:
- NOT NULL - there is no NULL value contained in a column.
- UNIQUE - every row in a specific column needs a unique value.
- PRIMARY KEY - a combination of UNIQUE and NOT NULL: columns must have an identity which is unique. This helps when you need to locate a specific data record faster.
- FOREIGN KEY - one table's SQL referential integrity matches another table's.
- CHECK - a defined condition is met within the value.
- DEFAULT - stock column value.
SQL Constraint: Summary
- You can define a constraint when you create a table, or afterrwards.
- Constraints stop data record actions with data that breaks the constraint rule.
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