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A database index speeds up data retrieval, similar to a book's index, improving query performance.

SQL databases use structured query language with a predefined schema; NoSQL databases store schema-less data with flexible models.

A foreign key links two tables by referring to the primary key in another table, ensuring referential integrity.

ACID: Atomicity (indivisible transactions), Consistency (consistent state transitions), Isolation (independent transactions), Durability (persisted changes).

Optimize using indexes, efficient SQL, limiting data retrieval, analyzing query performance, and considering denormalization.

Normalization organizes data into separate tables to reduce redundancy and improve integrity, following normalization forms.

INNER JOIN returns matching rows from both tables; LEFT JOIN returns all rows from the left table and matching rows from the right.

Stored procedures are precompiled SQL statements for data manipulation and logic, used for repetitive tasks and improving performance.

Denormalization introduces redundancy for performance, useful in read-heavy scenarios at the expense of storage and complexity.

ORM simplifies database interactions and is language-agnostic. It can introduce performance overhead and may limit database features.