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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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