AI Computer Science Academy · Faisalabad

SQL & Databases

Primary keys

Task 39 of 56 SQL Section 4 · Changing data
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Primary keys

A primary key uniquely identifies each row.

Task

What is a primary key?

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What you start with 
SELECT COUNT(*) AS students, COUNT(DISTINCT id) AS unique_ids FROM students;
Syntax reference 
SELECT COUNT(*) AS students, COUNT(DISTINCT id) AS unique_ids FROM students;

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The sample database 
CREATE TABLE courses (id INTEGER PRIMARY KEY, title TEXT, monthly_fee INTEGER);
CREATE TABLE students (id INTEGER PRIMARY KEY, name TEXT, city TEXT, course_id INTEGER, joined TEXT);
CREATE TABLE fees (id INTEGER PRIMARY KEY, student_id INTEGER, month TEXT, amount INTEGER, paid INTEGER);

INSERT INTO courses VALUES (1,'Web Design',9000),(2,'Python',11667),(3,'JavaScript',11667),(4,'AI & Prompt Engineering',15000);
INSERT INTO students VALUES
 (1,'Sara','Faisalabad',2,'2026-01-12'),
 (2,'Ali','Faisalabad',1,'2026-01-20'),
 (3,'Hina','Jhumra',2,'2026-02-02'),
 (4,'Bilal','Faisalabad',4,'2026-02-15'),
 (5,'Ayesha','Sargodha',3,'2026-03-01'),
 (6,'Usman','Jhumra',1,'2026-03-11'),
 (7,'Zainab','Faisalabad',4,'2026-04-05'),
 (8,'Hamza','Faisalabad',3,'2026-04-19');
INSERT INTO fees VALUES
 (1,1,'2026-01',11667,1),(2,1,'2026-02',11667,1),
 (3,2,'2026-01',9000,1),(4,2,'2026-02',9000,0),
 (5,3,'2026-02',11667,1),(6,3,'2026-03',11667,0),
 (7,4,'2026-02',15000,1),(8,4,'2026-03',15000,1),
 (9,5,'2026-03',11667,0),
 (10,6,'2026-03',9000,1),(11,6,'2026-04',9000,0),
 (12,7,'2026-04',15000,0),
 (13,8,'2026-04',11667,1);
SQL 1 automatic check Section 4 · Changing data
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