How to set SQL candidate key

Methods to set candidate keys in SQL: Determine a unique identification column; create a primary key using the PRIMARY KEY constraint; add a unique constraint using the UNIQUE constraint; create a unique index. Setting candidate keys ensures data integrity, improves query performance, and prevents data duplication.

SQL Candidate Key Settings

Candidate keys are columns or combinations of columns that uniquely identifies each row in the table. In SQL, you can set the candidate keys by following the steps:

1. Determine the unique identification column

• Analyze the data in the table to find the columns or combinations of columns that can uniquely identify each row.

2. Create primary key constraints using CREATE TABLE statement

• In the CREATE TABLE statement, use the PRIMARY KEY constraint to specify the primary key column. For example:

 <code class="sql">CREATE TABLE customers ( customer_id INT PRIMARY KEY, name VARCHAR(255) NOT NULL );</code>

3. Add unique constraints using the ALTER TABLE statement

• If the table already exists, you can use the ALTER TABLE statement to add unique constraints. For example:

 <code class="sql">ALTER TABLE orders ADD CONSTRAINT UK_order_number UNIQUE (order_number);</code>

4. Create a unique index using the CREATE UNIQUE INDEX statement

• Creating a unique index can also enforce candidate key constraints. For example:

 <code class="sql">CREATE UNIQUE INDEX idx_customer_name ON customers (name);</code>

advantage:

• Ensure the integrity and accuracy of the data in the table.
• Improve query performance and quickly find data through candidate keys.
• Prevent data duplication.

Notes:

• A table can have multiple candidate keys.
• Candidate keys are not necessarily unique keys. For example, a table can have multiple clients with the same name but other properties different.
• The column of the candidate key cannot be NULL.

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