SQL · Theory

Left Join

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Left Join

If you can teach Left Join using student + marks, you know it. If you only know the heading, you don’t.

Keep Left Join small. Ananya should finish student + marks in a few lines, not a 40-line dump.

Without Left Join, a report gets messy and student + marks is hard to trust.

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

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

Left Join is not a lonely heading. Ananya ties it to student + marks.

One breath for Left Join, then student + marks, then missing ON → cartesian product. Sit down.

Diagram

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

Exam tip

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

Left 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;

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

Short notes

DefLeft Join — Ananya uses it for student + marks in a report.
RuleLeft Join → JOIN … ON s.roll = m.roll.
RememberLeft Join + the query planner (a report).
UseLeft Join in a report (student + marks).
TrapLeft Join — missing ON → cartesian product.
ExLeft 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;

Left Join

Left Join — sample query

Short notes

Questions

Left Join

Left Join — sample query

Short notes

Questions