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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 More
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 More
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 sec)The following example will change the given time in specified format ... Read More
It can be done by using ‘COMMENT’ keyword while modifying the column with ALTER TABLE command. For example if we want to add comment in column ‘id’ of table ‘testing’ then following query will do itmysql> 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 More
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 More
While adding columns to an existing table with the help of ALTER command we can specify the default value also.SyntaxAlter table table-name ADD (column-name datatype default data);ExampleIn the example below, with the help of ALTER Command, column ‘City’ is added with default value ‘DELHI’ to the table ‘Student’.mysql> Alter table Student ADD(City Varchar(10) Default 'DELHI'); Query OK, 5 rows affected (0.33 sec) Records: 5 Duplicates: 0 Warnings: 0Now from DESCRIBE command, we can check the default value of ‘City’ column.mysql> describe Student\g +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default ... Read More
We can use keyword CHANGE to modify the column/s of an existing table. With CHANGE keyword we can change the name of the column and its definition both. Its syntax would be a bit different from the syntax of ALTER TABLE with MODIFY keyword.SyntaxAlter table table_name CHANGE old_columnname1 new_columnname1 datatype, CHANGE old_columnname2 new_columnname2 datatype… CHANGE old_columnnameN new_columnname datatype);ExampleIn the example below, with the help of CHANGE keyword in ALTER Command, the name and size of the columns ‘City’ and ‘RollNo’ have been modified.mysql> Alter table Student CHANGE Rollno Id int, CHANGE City Place Varchar(10); Query OK, 5 rows affected (0.40 ... Read More
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
If we want to get more details about the column of an existing table then we need to use SHOW FULL COLUMNS statement. Consider the example below in which SHOW FULL COLUMNS statement has been applied on ‘Employee’ table and MySQL returns result set with some extra details like Collation, Privileges, and Comment, about the columns of the table −mysql> SHOW FULL COLUMNS FROM EMPLOYEE\G *************************** 1. row *************************** Field: Id Type: int(11) Collation: NULL Null: YES Key: Default: NULL Extra: Privileges: select, insert, update, references ... Read More
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