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Database Articles
Page 318 of 547
What is the concept of CTAS (CREATE TABLE AS SELECTED) in MySQL?
CTAS i.e. “Create Table AS Select” script is used to create a table from an existing table. It copies the table structure as well as data from the existing table. Consider the following example in which we have created a table named EMP_BACKUP from an already existing table named ‘Employee’mysql> Select * from Employee; +------+--------+ | Id | Name | +------+--------+ | 100 | Ram | | 200 | Gaurav | | 300 | Mohan | +------+--------+ 3 rows in set (0.00 sec)The query above shows the ...
Read MoreHow can I use MySQL INTERVAL keyword while extracting the part of the date?
With the help of following example we can understand that how we can use MySQL INTERVAL keyword with EXTRACT() function −mysql> Select StudentName, RegDate, EXTRACT(YEAR from RegDate+INTERVAL 2 year) AS 'Two Year Interval' from testing where StudentName = 'Gaurav'; +-------------+---------------------+-------------------+ | StudentName | RegDate | Two Year Interval | +-------------+---------------------+-------------------+ | Gaurav | 2017-10-29 08:48:33 | 2019 | +-------------+---------------------+-------------------+ 1 row in set (0.02 sec)Above query is showing how we can use INTERVAL keyword with EXTRACT() function used in MySQL table query.mysql> Select EXTRACT(Year from '2017-10-22 ...
Read MoreHow to use together the date and time format characters in MySQL DATE_FORMAT() function?
We can use both the format characters together in DATE_FORMAT() function. The following example will clarify this −mysql> SELECT DATE_FORMAT(NOW(), 'The time is %a %h:%i:%s:%f %p'); +-----------------------------------------------------+ | DATE_FORMAT(NOW(), 'The time is %a %h:%i:%s:%f %p') | +-----------------------------------------------------+ | The time is Sun 06:35:06:000000 AM | +-----------------------------------------------------+ 1 row in set (0.00 sec)Above query is using date format character ‘%a’ along with other time format characters.Following is another example in which both format characters are used together −mysql> SELECT DATE_FORMAT(NOW(), 'The date & time is %a %D %M %Y %h:%i:%s:%f %p'); ...
Read MoreHow to get the total number of seconds from a MySQL DATETIME instance?
The MySQL DateTime instance can be converted into seconds with the help of UNIX_TIMESTAMP() function in the following way −mysql> Select UNIX_TIMESTAMP('2017-05-15 04:05:30') AS 'NUMBER OF SECONDS'; +-------------------+ | NUMBER OF SECONDS | +-------------------+ | 1494801330 | +-------------------+ 1 row in set (0.00 sec)Above query will convert the given datetime instance into total number of seconds.mysql> Select UNIX_TIMESTAMP(NOW()) AS 'NUMBER OF SECONDS'; +-------------------+ | NUMBER OF SECONDS | +-------------------+ | 1509248856 | +-------------------+ 1 row in set (0.00 sec)Above query will convert the current DateTime instance into a total number of seconds.mysql> ...
Read MoreHow can we use MySQL ALTER TABLE command for adding comments on columns?
We can use ‘COMMENT’ keyword with ALTER TABLE command while modifying the column to add comments on columns. For example if we want to add comment in column ‘id’ of table ‘testing’ then following query will do it −mysql> ALTER TABLE testing MODIFY id INT COMMENT 'id of employees'; Query OK, 0 rows affected (0.07 sec) Records: 0 Duplicates: 0 Warnings: 0With following query it can be checked in the comment field of a column.mysql> Show full columns from testing\G *************************** 1. row *************************** Field: id Type: int(11) Collation: NULL Null: NO ...
Read MoreWhat would be output if we will try to extract time values by providing the date values only to MySQL EXTRACT() function?
When we try to extract hour value from a date, then EXTRACT() function will give the output 0 with a warning as shown in the below-given example −mysql> Select EXTRACT(Hour from '2017-10-20'); +---------------------------------+ | EXTRACT(Hour from '2017-10-20') | +---------------------------------+ | 0 | +---------------------------------+ 1 row in set, 1 warning (0.00 sec) mysql> Show Warnings; +---------+------+----------------------------------------------+ | Level | Code | Message | +---------+------+----------------------------------------------+ ...
Read MoreIn MySQL, how can we display time in other format specified by the user?
We can also use DATE_FORMAT() function to display the time in other formats. In this case, there would be two arguments of this function, first would be the time and second would be the format string.The following example will change the current time in specified format −mysql> SELECT DATE_FORMAT(NOW(), 'The time is %h:%i:%s %p'); +-----------------------------------------------+ | DATE_FORMAT(NOW(), 'The time is %h:%i:%s %p') | +-----------------------------------------------+ | The time is 06:02:28 AM | +-----------------------------------------------+ 1 row in set (0.00 ...
Read MoreHow can I change the value of an instance of a row in MySQL table?
UPDATE command along with WHERE clause can be used to change the value of an instance of a row. Basically, MySQL will change the value on the basis of the condition given in the query. Following example can demonstrate itSuppose we want to change the name from ‘Ram’ to ‘Mohit’ in the ‘testing’ table given below −mysql> Select * from testing; +----+---------+ | Id | Name | +----+---------+ | 1 | Harshit | | 2 | Lovkesh | | 3 | Ram | | 4 | Gaurav | +----+---------+ 4 rows in set (0.00 sec)Now ...
Read MoreWhat MySQL returns when we remove all the columns from a table by using ALTER TABLE command with DROP keyword?
Eventually, we cannot remove all the columns from a table by using ALTER TABLE command with DROP keyword. In this case, MySQL will return an error message. It is demonstrated with the help of the following exampleSuppose in table ‘Employee’ we have two columns ‘name’ and ‘id’, now if we will use ALTER TABLE to remove both the columns then MySQL returns an error as followsmysql> ALTER TABLE employee drop column name, drop column id; ERROR 1090 (42000): You can't delete all columns with ALTER TABLE; use DROP TABLE instead
Read MoreWhy MySQL uses the interval like 7 day and 2 hour instead of 7 days and 2 hours?
The reason behind this concept is that MySQL requires the unit keywords to be singular, regardless of the English grammar rules. If we will try to supply intervals like 7 days, 2 hours etc then MySQL will produce syntax error as follows −mysql> Select '2017-02-25 05:04:30' + INTERVAL 2 days; ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'days' at line 1
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