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My SQL Interview Questions and Answers


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.

If you specify the data type as DECIMAL (5,2), what’s the range of values that can go in this table?
999.99 to -99.99. Note that with the negative number the minus sign is considered one of the digits.

What happens if a table has one column defined as TIMESTAMP?
That field gets the current timestamp whenever the row gets altered.

But what if you really want to store the timestamp data, such as the publication date of the article?
Create two columns of type TIMESTAMP and use the second one for your real data.

Explain data type TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP ?
The column exhibits the same behavior as a single timestamp column in a table with no other timestamp columns.

What does TIMESTAMP ON UPDATE CURRENT_TIMESTAMP data type do?
On initialization places a zero in that column, on future updates puts the current value of the timestamp in.

Explain TIMESTAMP DEFAULT ‘2006:09:02 17:38:44? ON UPDATE CURRENT_TIMESTAMP. ?
A default value is used on initialization, a current timestamp is inserted on update of the row.

If I created a column with data type VARCHAR(3), what would I expect to see in MySQL table?
CHAR(3), since MySQL automatically adjusted the data type.

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