In SQL, aprimary key is explicitly defined using the PRIMARY KEY keywordwhen creating a table.

Example Usage:

sql

CREATE TABLE Products (

ProductID INT PRIMARY KEY,

Name VARCHAR(100),

Price DECIMAL(10,2)

);

Here,PRIMARY KEY is the SQL keyword that designates ProductID as the primary key.

Why Other Options Are Incorrect:

Option A (Formula in SQL) (Incorrect):SQLdoes not use formulas to define primary keys.

Option C (Bold typeface) (Incorrect):SQL syntax does not rely on text formatting.

Option D (Diamond symbol) (Incorrect):ER diagramsmight use symbols, but SQLdoes not use diamonds to indicate keys.

Thus, the correct answer isSQL keyword, as primary keys are explicitly defined using PRIMARY KEY.