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Union vs Join

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Union vs Join

Don’t start Union vs Join with jargon. Start with student + marks. That is the whole point.

Keep Union vs Join small. Vikram should finish student + marks in a few lines, not a 40-line dump.

Union vs Join exists so Vikram can keep student + marks correct in a report.

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

Union vs Join trap — missing ON → cartesian product. Vikram loses marks for that every viva.

Union vs Join is not a lonely heading. Vikram ties it to student + marks.

Viva for Union vs 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 Union vs Join in one breath, then JOIN … ON s.roll = m.roll, then missing ON → cartesian product.

Union vs 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;

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

Short notes

DefUnion vs Join — Vikram uses it for student + marks in a report.
RuleUnion vs Join → JOIN … ON s.roll = m.roll.
RememberUnion vs Join + an index (a report).
UseUnion vs Join in a report (student + marks).
TrapUnion vs Join — missing ON → cartesian product.
ExUnion vs 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;

Union vs Join

Union vs Join — sample query

Short notes

Questions

Union vs Join

Union vs Join — sample query

Short notes

Questions