SQL · Theory

Left Join vs Right Join

Theory

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Left Join vs Right Join

LEFT keeps the left table complete. RIGHT keeps the right table complete. Same power. Teams pick LEFT and put the important table first.

Example: all students + marks if any → students LEFT JOIN marks. All marks + student name if any → marks LEFT JOIN students (or students RIGHT JOIN marks).

Trap — saying they return the same rows without swapping tables.

On the example next to this theory: Left Join vs Right Join — jOIN combines rows using a match condition between tables.

Diagram

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

Exam tip

LEFT vs RIGHT = which table is mandatory. Prefer LEFT.

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

Short notes

DefWhich side is kept in full — left vs right.
RulePrefer LEFT JOIN only. Swap table order.
TrapClaiming LEFT and RIGHT are identical without swapping.

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 vs Right Join

Left Join vs Right Join — sample query

Short notes

Questions

Left Join vs Right Join

Left Join vs Right Join — sample query

Short notes

Questions