DBA > Interview Resource

MySQL and SQL

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What are HEAP tables in MySQL?

HEAP tables are in-memory. They are usually used for high-speed temporary storage. No TEXT or BLOB fields are allowed within HEAP tables. You can only use the comparison operators = and <=>. HEAP tables do not support AUTO_INCREMENT. Indexes must be NOT NULL.


How do you control the max size of a HEAP table?

MySQL config variable max_heap_table_size.


What are CSV tables?

Those are the special tables, data for which is saved into comma-separated values files. They cannot be indexed.


Explain federated tables.

Introduced in MySQL 5.0, federated tables allow access to the tables located on other databases on other servers.


What is SERIAL data type in MySQL?

BIGINT NOT NULL PRIMARY KEY AUTO_INCREMENT


What happens when the column is set to AUTO INCREMENT and you reach the maximum value for that table?

It stops incrementing. It does not overflow to 0 to prevent data losses, but further inserts are going to produce an error, since the key has been used already.


Explain the difference between BOOL, TINYINT and BIT.

Prior to MySQL 5.0.3: those are all synonyms. After MySQL 5.0.3: BIT data type can store 8 bytes of data and should be used for binary data.


Explain the difference between FLOAT, DOUBLE and REAL.

FLOATs store floating point numbers with 8 place accuracy and take up 4 bytes. DOUBLEs store floating point numbers with 16 place accuracy and take up 8 bytes. REAL is a synonym of FLOAT for now.

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