SQL · Theory

WHERE

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WHERE

WHERE is a SQL idea you prove with students who failed, not with a slogan.

Keep WHERE small. Riya should finish students who failed in a few lines, not a 40-line dump.

Skip WHERE and WHERE marks = NULL shows up in the result sheet.

Use WHERE when students who failed must stay clear. If a simpler SQL step works, use that instead.

Don’t do this with WHERE — WHERE marks = NULL. Interviewers spot it in ten seconds.

After WHERE, Riya should still remember WHERE marks = NULL.

Close WHERE with — “If I skip it, students who failed goes wrong like this: WHERE marks = NULL.”

Diagram

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

Exam tip

Say WHERE in one breath, then WHERE marks < 40, then WHERE marks = NULL.

WHERE — sample query

CREATE TABLE products (id INTEGER PRIMARY KEY, name TEXT, price REAL);
INSERT INTO products VALUES (1, 'Pen', 10), (2, 'Book', 200), (3, 'Bag', 50);
SELECT * FROM products
WHERE price BETWEEN 20 AND 150 AND name LIKE 'B%';

WHERE combines predicates with AND/OR; LIKE does pattern match.

Short notes

DefWHERE — Riya uses it for students who failed in the result sheet.
RuleWHERE → WHERE marks < 40.
RememberWHERE + the query planner (the result sheet).
UseWHERE in the result sheet (students who failed).
TrapWHERE — WHERE marks = NULL.
ExWHERE → students who failed.

CREATE TABLE products (id INTEGER PRIMARY KEY, name TEXT, price REAL); INSERT INTO products VALUES (1, 'Pen', 10), (2, 'Book', 200), (3, 'Bag', 50); SELECT * FROM products WHERE price BETWEEN 20 AND 150 AND name LIKE 'B%';

WHERE

WHERE — sample query

Short notes

Questions

WHERE

WHERE — sample query

Short notes

Questions