SQL Interview Questions for Freshers

WHERE filters rows before grouping and cannot use aggregate functions. HAVING filters groups after GROUP BY and can use aggregates such as COUNT or SUM.

INNER JOIN returns only rows that match in both tables. LEFT JOIN returns every row of the left table and fills the right table's columns with NULL where there is no match.

A primary key identifies each row uniquely and cannot be NULL. A unique key also stops duplicates but allows a NULL. A foreign key points to a primary key in another table to link them.

Organising tables to reduce repeated data and mistakes. First normal form has single values in each cell, second removes partial dependencies and third removes dependencies between non-key columns.

DELETE removes chosen rows and can be rolled back. TRUNCATE removes all rows quickly and keeps the table. DROP removes the whole table and its structure.

An index is a structure that makes searches on a column faster. It speeds up reads but slows inserts and updates and uses space, so index the columns you filter and join on, not everything.

Atomicity (all or nothing), Consistency (rules always hold), Isolation (transactions do not disturb each other) and Durability (committed data survives a crash). They keep transactions reliable.

One way is to select the maximum salary below the overall maximum, using a subquery. Another is to use DENSE_RANK or LIMIT with OFFSET. Mention duplicates and what you want when two people share a salary.

A subquery is a query inside another, a join combines tables side by side. Joins are often faster and clearer, while subqueries can be easier to read for simple filters.

A saved query that behaves like a virtual table. It hides complexity, can restrict what users see and does not store data itself, unless it is a materialised view.