SQL · Theory

Update Join

Theory

66/439

Update Join

UPDATE students s JOIN marks m ON s.roll = m.roll SET s.marks = m.score WHERE m.subject = 'Maths'; copy a value across tables. JOIN picks the rows, SET changes them.

Preview with SELECT s.roll, s.marks, m.score FROM … the same JOIN. Then UPDATE.

Trap: UPDATE without JOIN when the new value lives in another table. Trap: multiple marks rows per student — which one wins is messy; be specific in WHERE.

On the example next to this theory — Update Join — uPDATE changes existing rows — always scope with WHERE.

Diagram

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

Exam tip

UPDATE s JOIN m ON s.roll = m.roll SET s.marks = m.score;

Update Join — sample query

CREATE TABLE products (id INTEGER PRIMARY KEY, name TEXT, price REAL);
INSERT INTO products VALUES (1, 'Mouse', 499);
UPDATE products SET price = 449 WHERE id = 1;
SELECT * FROM products;

Update Join — uPDATE changes existing rows — always scope with WHERE.

Short notes

DefUPDATE … JOIN … SET … updates using another table.
RuleSame JOIN as the preview SELECT.
TrapOne student matching many marks rows.

SQL · Theory

Update Join

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Theory

66/439

Update Join

UPDATE students s JOIN marks m ON s.roll = m.roll SET s.marks = m.score WHERE m.subject = 'Maths'; copy a value across tables. JOIN picks the rows, SET changes them.

Preview with SELECT s.roll, s.marks, m.score FROM … the same JOIN. Then UPDATE.

Trap: UPDATE without JOIN when the new value lives in another table. Trap: multiple marks rows per student — which one wins is messy; be specific in WHERE.

On the example next to this theory — Update Join — uPDATE changes existing rows — always scope with WHERE.

Diagram

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

Exam tip

UPDATE s JOIN m ON s.roll = m.roll SET s.marks = m.score;

Update Join — sample query

CREATE TABLE products (id INTEGER PRIMARY KEY, name TEXT, price REAL);
INSERT INTO products VALUES (1, 'Mouse', 499);
UPDATE products SET price = 449 WHERE id = 1;
SELECT * FROM products;

Update Join — uPDATE changes existing rows — always scope with WHERE.

Short notes

DefUPDATE … JOIN … SET … updates using another table.
RuleSame JOIN as the preview SELECT.
TrapOne student matching many marks rows.

Update Join

Update Join — sample query

Short notes

Questions

Update Join

Update Join — sample query

Short notes

Questions