Not Exists — SQL Technical Interview Questions & Answers

Not Exists

Not Exists

  • Not Exists is a MySQL topic you learn with one small college example — a students table with roll, name, marks. Say what Not Exists does in one sentence, then write the shortest SQL that shows it.
  • Pattern: USE college; then a CREATE/SELECT/UPDATE that needs Not Exists. If it is a function, SELECT it on one column. If it is DDL, run SHOW / DESCRIBE after so you can see the change.
  • Trap: only reciting the heading Not Exists with no SQL. Interviewers ask you to type. Another trap: mixing this up with a neighbouring command (DELETE vs TRUNCATE vs DROP, WHERE vs HAVING, JOIN vs UNION).
  • Keep the example on paper: two or three rows. Change one value. Predict the result before you run it.
  • On the example next to this theory: Not Exists — uNION merges result sets and drops duplicate rows.

Technical Interview Tip: What is Not Exists? One sentence + one tiny SQL on students.

Code Examples for Not Exists

Not Exists — sample query

CREATE TABLE a (id INTEGER, label TEXT);
CREATE TABLE b (id INTEGER, label TEXT);
INSERT INTO a VALUES (1, 'x'), (2, 'y');
INSERT INTO b VALUES (2, 'y'), (3, 'z');
SELECT label FROM a
UNION
SELECT label FROM b;

Not Exists — uNION merges result sets and drops duplicate rows.

How to Explain Not Exists to an Interviewer

What is Not Exists?

A MySQL idea used with tables/queries Show one short SQL

Give an example.

students table SQL that uses Not Exists

What mistake do beginners make?

Name only, no SQL Mixing nearby commands