SQL · Theory

Inner Join

← All stacks

Theory

60/439

Inner Join

Meera only reaches for Inner Join when student + marks has to stay correct in a report.

Inner Join on the board — JOIN … ON s.roll = m.roll. Then say what student + marks looks like after.

Inner Join exists so Meera can keep student + marks correct in a report.

Use Inner Join when student + marks must stay clear. If a simpler SQL step works, use that instead.

If student + marks breaks under Inner Join, check missing ON → cartesian product first.

Inner Join is not a lonely heading. Meera ties it to student + marks.

Viva for Inner Join — what it is → JOIN … ON s.roll = m.roll → the mistake (missing ON → cartesian product).

Diagram

  students.id
       =
  marks.student_id
        │
        ▼
     one result row

Exam tip

Say Inner Join in one breath, then JOIN … ON s.roll = m.roll, then missing ON → cartesian product.

Inner Join — sample query

CREATE TABLE customers (id INTEGER PRIMARY KEY, name TEXT);
CREATE TABLE orders (id INTEGER PRIMARY KEY, customer_id INTEGER, total INTEGER);
INSERT INTO customers VALUES (1, 'Asha'), (2, 'Ravi');
INSERT INTO orders VALUES (10, 1, 500), (11, 1, 200);
SELECT c.name, o.total
FROM customers c
JOIN orders o ON o.customer_id = c.id;

Inner Join — jOIN combines rows using a match condition between tables.

Short notes

DefInner Join — Meera uses it for student + marks in a report.
RuleInner Join → JOIN … ON s.roll = m.roll.
RememberInner Join + InnoDB (a report).
UseInner Join in a report (student + marks).
TrapInner Join — missing ON → cartesian product.
ExInner Join → student + marks.

CREATE TABLE customers (id INTEGER PRIMARY KEY, name TEXT); CREATE TABLE orders (id INTEGER PRIMARY KEY, customer_id INTEGER, total INTEGER); INSERT INTO customers VALUES (1, 'Asha'), (2, 'Ravi'); INSERT INTO orders VALUES (10, 1, 500), (11, 1, 200); SELECT c.name, o.total FROM customers c JOIN orders o ON o.customer_id = c.id;

Inner Join

Inner Join — sample query

Short notes

Questions

Inner Join

Inner Join — sample query

Short notes

Questions