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MySQL Articles
Page 353 of 355
How can we remove FOREIGN KEY constraint from a column of an existing MySQL table?
We can remove FOREIGN KEY constraint from a column of an existing table by using DROP keyword along with ALTER TABLE statement. Syntax ALTER TABLE table_name DROP FOREIGN KEY constraint_name Here constraint name is the name of foreign key constraint which we applied while creating the table. If no constraint name is specified then MySQL will provide constraint name which can be checked by SHOW CREATE TABLE statement. Example The following query will delete the FOREIGN KEY constraint from ‘orders’ table − mysql> Alter table orders DROP FOREIGN KEY orders_ibfk_1; Query OK, 0 rows affected (0.22 sec) ...
Read MoreHow can we apply AUTO_INCREMENT to a column?
AUTO_INCREMENT means that the column will get the value automatically. To illustrate it we have created a table ‘employees’ as follows − mysql> Show Create Table employees\G *************************** 1. row *************************** Table: employees Create Table: CREATE TABLE `employees` ( `Id` int(11) NOT NULL AUTO_INCREMENT, `Name` varchar(35) DEFAULT NULL, PRIMARY KEY (`Id`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1 1 row in set (0.00 sec) From the above result set, we can see that column id is given the auto-increment option. Now, when we will insert the value in Name ...
Read MoreHow can we check that by default MySQL CHAR() function returns a binary string?
With the help of CHARSET() function, we can check which string is returned by MySQL CHAR() function. Following result set will demonstrate it − mysql> Select CHARSET(CHAR(85)); +-------------------+ | CHARSET(CHAR(85)) | +-------------------+ | binary | +-------------------+ 1 row in set (0.00 sec)
Read MoreHow does the value of system variable max_allowed_packet affect the result of a string-valued function?
String-valued functions return NULL if the length of the result would be greater than the value of the max_allowed_packet system variable. Actually, max_allowed_packet is a dynamic global variable which can accept the integer type values. These values can be set for a session only. It can accept 1024 as the minimum value and 1073741824 as the maximum value. The by the default value of this system variable is 1048576.
Read MoreWhat are the drawbacks of using test database?
There is a database named test in the list of databases displayed by the statement SHOW DATABASES. We can use test database but the main disadvantage is that anything created in this database can be removed/changed by anyone else with access to it. To avoid this we should take permission from MySQL administrator to use a database of our own. For taking permission following command must be run − mysql> grant all on tutorial.* to root@localhost; Query OK, 0 rows affected (0.10 sec) In the above command, I am taking permission for the tutorial database. Root is the ...
Read MoreHow MySQL behaves if I use INTERVAL keyword with an invalid date?
Actually, the behavior of MySQL depends upon the allow_invalid_dates mode. If this mode is enabled then MySQL will accept the invalid date and it would perform the date arithmetic as it performs with a valid date. Otherwise, if this mode is inactive then it will not accept the invalid date and would produce NULL as output. mysql> select '2017-02-30' + INTERVAL 7 day; +-------------------------------+ | '2017-02-30' + INTERVAL 7 day | +-------------------------------+ | NULL ...
Read MoreWhat happens to MySQL temporary tables if MySQL session is ended?
Temporary table would be deleted if MySQL session terminates. After login again, on issuing the SELECT command we will find no data available in the database. Even our temporary table will not exist.
Read MoreWhat happens if the output of MySQL TIMEDIFF() function surpass the range value of TIME field?
As we know that the range of TIME field in MySQL is ‘-838:59:59’ to ‘838:59:59’. Now, if TIMEDIFF() function’s output surpasses this range then MySQL will return either ‘-838:59:59’ or ‘838:59:59’ depends upon the values of the argument. Example mysql> Select TIMEDIFF('2017-09-01 03:05:45', '2017-10-22 03:05:45')AS 'Out of Range TIME Difference'; +------------------------------+ | Out of Range TIME Difference | +------------------------------+ | -838:59:59 | +------------------------------+ 1 row in set, 1 warning (0.00 sec) mysql> Select TIMEDIFF('2017-10-22 04:05:45', '2017-09-01 03:05:45')AS 'Out of Range ...
Read MoreWhat would be the effect on MySQL output if we have the combination of NULL and other values in the list of strings, provided as arguments in FIELD() function?
There will be a significant change in the output if we have the combination of NULL and other values in the list of strings, provided as arguments in FIELD() function. Following example will demonstrate it Example mysql> Select FIELD('good', 'Ram', 'is', 'good', 'boy'); +---------------------------------------+ | FIELD('good', 'Ram', 'is', 'good', 'boy') | +---------------------------------------+ | 3 | +---------------------------------------+ 1 row in set (0.00 sec) ...
Read MoreWhat happens if a NULL argument is provided in MySQL CONV() function?
MySQL will return NULL as the output if any of the argument of CONV() function is NULL or if the value provided for the base is out of limit(i.e. not between minimum 2 and maximum 36). Following examples would demonstrate it. Example mysql> Select CONV(10,NULL,2); +-----------------+ | CONV(10, NULL,2)| +-----------------+ | NULL | +-----------------+ 1 row in set (0.00 sec) mysql> Select CONV(10,10, NULL); +------------------+ | CONV(10,10, NULL)| +------------------+ | NULL | +------------------+ 1 row in set (0.00 sec) mysql> Select CONV(NULL,10,2); +-----------------+ | CONV(null,10,2) | +-----------------+ | NULL | +-----------------+ 1 row in set (0.00 sec)
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