SQL · Theory

FROM

Theory

42/439

FROM

FROM students names the table (or view). SELECT cannot guess the table. Joins also live here: FROM students s JOIN marks m ON s.roll = m.roll.

Alias — FROM students AS s — s.name is shorter. Needed when two tables share column names.

Trap — FROM missing. Trap: FROM students, marks without ON — old comma join = CROSS JOIN.

On the example next to this theory — FROM — sELECT projects columns; WHERE filters; ORDER BY sorts.

Diagram

  FROM students
        │
        ▼
  WHERE city = 'Pune'
        │
        ▼
  SELECT name, marks

Exam tip

FROM students. Joins belong in FROM … ON …

FROM — sample query

CREATE TABLE employees (id INTEGER PRIMARY KEY, name TEXT, dept TEXT, salary INTEGER);
INSERT INTO employees VALUES
  (1, 'Asha', 'Eng', 70000),
  (2, 'Ravi', 'Eng', 65000),
  (3, 'Neha', 'HR', 50000);
SELECT name, salary FROM employees WHERE dept = 'Eng' ORDER BY salary DESC;

FROM — sELECT projects columns; WHERE filters; ORDER BY sorts.

Short notes

DefFROM picks the table or join.
RuleAlias tables in joins (s, m).
TrapComma join without ON.

SQL · Theory

FROM

← All stacks

Theory

42/439

FROM

FROM students names the table (or view). SELECT cannot guess the table. Joins also live here: FROM students s JOIN marks m ON s.roll = m.roll.

Alias — FROM students AS s — s.name is shorter. Needed when two tables share column names.

Trap — FROM missing. Trap: FROM students, marks without ON — old comma join = CROSS JOIN.

On the example next to this theory — FROM — sELECT projects columns; WHERE filters; ORDER BY sorts.

Diagram

  FROM students
        │
        ▼
  WHERE city = 'Pune'
        │
        ▼
  SELECT name, marks

Exam tip

FROM students. Joins belong in FROM … ON …

FROM — sample query

CREATE TABLE employees (id INTEGER PRIMARY KEY, name TEXT, dept TEXT, salary INTEGER);
INSERT INTO employees VALUES
  (1, 'Asha', 'Eng', 70000),
  (2, 'Ravi', 'Eng', 65000),
  (3, 'Neha', 'HR', 50000);
SELECT name, salary FROM employees WHERE dept = 'Eng' ORDER BY salary DESC;

FROM — sELECT projects columns; WHERE filters; ORDER BY sorts.

Short notes

DefFROM picks the table or join.
RuleAlias tables in joins (s, m).
TrapComma join without ON.

FROM

FROM — sample query

Short notes

Questions

FROM

FROM — sample query

Short notes

Questions