A self join is used to:

Join a table to itself to compare rows within the same table

A SEMI JOIN (e.g., EXISTS) returns:

Rows from the left for which at least one match exists on the right, without duplicating left rows

Rows from the left, duplicated for each match on the right

With OUTER JOINS, putting a right-table filter in the WHERE clause typically:

Converts the query to INNER JOIN semantics

In a LEFT JOIN, adding WHERE right.col IS NOT NULL effectively:

Turns the logic into an INNER JOIN on right.col

Removes duplicates from the result

Joining can introduce duplicates primarily because:

Many-to-many relationships replicate left rows for each match

The optimizer always duplicates rows

Foreign key constraints require duplicates

For INNER JOINS, which property holds?

Neither associative nor commutative

16. Which statement about OUTER JOINS is true?

OUTER JOINS return all rows from both tables, matching rows where possible.

OUTER JOINS only return rows where there is a match in both tables.

OUTER JOINS exclude rows with NULL values.

OUTER JOINS are used only for combining identical tables.

The USING clause in a JOIN:

Matches columns by name and outputs a single merged column for the join key

Specifies different column names to match

Nested-loop join is typically best when:

One input is small and/or there is an index on the larger table's join key

Both inputs are very large and unsorted

Join keys are hashed into memory

Hash join is typically preferred when:

Equality condition and large inputs (fit in memory or can be partitioned)

Sort-merge join is especially useful when:

An inequality join is required or both inputs are pre-sorted on join keys

Only hashable types are allowed

To find rows in A that have no match in B, use:

B. A LEFT JOIN B ON ... WHERE B.key IS NULL

Which predicate avoids NULL pitfalls for anti-joins?

NOT IN (subquery) regardless of NULLs

NATURAL JOIN can be dangerous because:

It implicitly uses all same-named columns, which may change after schema updates

A self join generally requires:

Table aliases to distinguish instances of the same table

The USING clause in SELECT ... JOIN ... USING(col):

Produces one merged column for the join key

Produces two separate columns, both named col

FULL OUTER JOIN can be simulated by:

UNION of LEFT JOIN and RIGHT JOIN results with appropriate filters

Join elimination is when the optimizer:

Removes a join that does not affect results due to constraints

In a star schema, the common pattern is:

Fact table INNER JOIN to dimensions on foreign keys

Fact table CROSS JOIN all dimensions

Dimensions outer join each other

LEFT OUTER JOIN followed by WHERE right.col > 100 usually:

Removes unmatched rows (behaves like INNER JOIN for that condition)

38. Which query is a SEMI JOIN?

SELECT * FROM A WHERE EXISTS (SELECT 1 FROM B WHERE B.k = A.k)

SELECT * FROM A LEFT JOIN B ON A.k = B.k

SELECT * FROM B WHERE EXISTS (SELECT 1 FROM A WHERE A.k = B.k)

Which query expresses an ANTI JOIN?

SELECT * FROM A WHERE NOT EXISTS (SELECT 1 FROM B WHERE B.k = A.k)

SELECT * FROM A WHERE EXISTS (SELECT 1 FROM B WHERE B.k = A.k)

SELECT * FROM A INNER JOIN B ON A.k = B.k

For INNER JOINS, placing an additional filter in the ON clause versus the WHERE clause is:

Equivalent (differences only matter for OUTER JOINs)

Composite joins require:

Equality on multiple columns across the tables

Which query counts A-rows with no match in B?

SELECT COUNT(*) FROM A LEFT JOIN B ON A.k = B.k WHERE B.k IS NULL

SELECT COUNT(*) FROM A INNER JOIN B ON A.k = B.k

SELECT COUNT(*) FROM A CROSS JOIN B

SELECT COUNT(*) FROM A NATURAL JOIN B

For existence/anti-existence checks, which is generally preferable?

NOT EXISTS (SELECT 1 FROM B WHERE B.k = A.k)

LEFT JOIN ... WHERE B.k IS NULL

Join selectivity refers to:

The fraction of row pairs that match across join keys

The size of the cartesian product

A foreign key from A.k to B.k implies:

Every non-NULL A.k must match some B.k (enabling possible join elimination)

The DBMS automatically performs joins

An equi-join A.k = B.k where both sides are unique means:

At most one B per A and one A per B

DBMS Joins – 60 MCQs

Authored by Rather Rather

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2nd Grade

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MULTIPLE CHOICE QUESTION

30 sec • 1 pt

Which join returns rows that have matching values in both tables?

LEFT OUTER JOIN

INNER JOIN

RIGHT OUTER JOIN

FULL OUTER JOIN

MULTIPLE CHOICE QUESTION

30 sec • 1 pt

A LEFT OUTER JOIN returns:

Only matching rows

All rows from the left table and matching rows from the right

All rows from the right table and matching rows from the left

All rows from both tables regardless of match

MULTIPLE CHOICE QUESTION

30 sec • 1 pt

A RIGHT OUTER JOIN returns:

Only matching rows

All rows from the left table and matching rows from the right

All rows from the right table and matching rows from the left

All rows from both tables regardless of match

MULTIPLE CHOICE QUESTION

30 sec • 1 pt

A FULL OUTER JOIN returns:

Only matching rows

Only unmatched rows

Only rows from the larger table

All rows from both tables with NULLs for missing matches

MULTIPLE CHOICE QUESTION

30 sec • 1 pt

A CROSS JOIN between tables with m and n rows produces:

m + n rows

m × n rows

max(m, n) rows

min(m, n) rows

MULTIPLE CHOICE QUESTION

30 sec • 1 pt

A NATURAL JOIN:

Joins on columns with the same name and removes one duplicate column

Automatically joins on primary keys only

Requires a USING clause

Performs a cross product then filters duplicates

MULTIPLE CHOICE QUESTION

30 sec • 1 pt

Equijoin vs NATURAL JOIN — which is correct?

They are identical in all SQL dialects

NATURAL JOIN enforces equality on same-named columns and removes duplicate output columns

Equijoin requires a USING clause

NATURAL JOIN supports inequality predicates by default

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DBMS Joins – 60 MCQs

Which join returns rows that have matching values in both tables?

A LEFT OUTER JOIN returns:

A RIGHT OUTER JOIN returns:

A FULL OUTER JOIN returns:

A CROSS JOIN between tables with m and n rows produces:

A NATURAL JOIN:

Equijoin vs NATURAL JOIN — which is correct?

A theta join is:

A self join is used to:

A SEMI JOIN (e.g., EXISTS) returns:

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