SQL · Theory

Not Exists

Theory

419/439

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.

Exam tip

What is Not Exists? One sentence + one tiny SQL on students.

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.

Short notes

DefNot Exists — explain with students(roll, name, marks).
RuleShortest SQL that uses Not Exists. Then SHOW/SELECT the effect.
TrapHeading only, no query. Mixing nearby commands.

SQL · Theory

Not Exists

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Theory

419/439

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.

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.

Exam tip

What is Not Exists? One sentence + one tiny SQL on students.

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.

Short notes

DefNot Exists — explain with students(roll, name, marks).
RuleShortest SQL that uses Not Exists. Then SHOW/SELECT the effect.
TrapHeading only, no query. Mixing nearby commands.

Not Exists

Not Exists — sample query

Short notes

Questions

Not Exists

Not Exists — sample query

Short notes

Questions