Given the table created using:
CREATE TABLE tbl_Country
(
CountryId INT NOT NULL AUTO_INCREMENT,
IsDeleted bit,
PRIMARY KEY (CountryId)
)
How can I delete the column IsDeleted?
ALTER TABLE tbl_Country DROP COLUMN IsDeleted;
Here's a working example.
Note that the COLUMN keyword is optional, as MySQL will accept just DROP IsDeleted. Also, to drop multiple columns, you have to separate them by commas and include the DROP for each one.
ALTER TABLE tbl_Country
DROP COLUMN IsDeleted,
DROP COLUMN CountryName;
This allows you to DROP, ADD and ALTER multiple columns on the same table in the one statement. From the MySQL reference manual:
You can issue multiple ADD, ALTER, DROP, and CHANGE clauses in a single ALTER TABLE statement, separated by commas. This is a MySQL extension to standard SQL, which permits only one of each clause per ALTER TABLE statement.
Use ALTER TABLE with DROP COLUMN to drop a column from a table, and CHANGE or MODIFY to change a column.
ALTER TABLE tbl_Country DROP COLUMN IsDeleted;
ALTER TABLE tbl_Country MODIFY IsDeleted tinyint(1) NOT NULL;
ALTER TABLE tbl_Country CHANGE IsDeleted IsDeleted tinyint(1) NOT NULL;
To delete a single column:
ALTER TABLE `table1` DROP `column1`;
To delete multiple columns:
ALTER TABLE `table1`
DROP `column1`,
DROP `column2`,
DROP `column3`;
You can use
alter table <tblname> drop column <colname>
ALTER TABLE `tablename` DROP `columnname`;
Or,
ALTER TABLE `tablename` DROP COLUMN `columnname`;
If you are running MySQL 5.6 onwards, you can make this operation online, allowing other sessions to read and write to your table while the operation is been performed:
ALTER TABLE tbl_Country DROP COLUMN IsDeleted, ALGORITHM=INPLACE, LOCK=NONE;
Use ALTER:
ALTER TABLE `tbl_Country` DROP COLUMN `column_name`;
ALTER TABLE tbl_Country DROP columnName;
It is worth mentioning that MySQL 8.0.23 and above supports Invisible Columns
CREATE TABLE tbl_Country(
CountryId INT NOT NULL AUTO_INCREMENT,
IsDeleted bit,
PRIMARY KEY (CountryId)
);
INSERT INTO tbl_Country VALUES (1, 1), (2,0);
ALTER TABLE tbl_Country ALTER COLUMN IsDeleted SET INVISIBLE;
SELECT * FROM tbl_Country;
CountryId
1
2
ALTER TABLE tbl_Country DROP COLUMN IsDeleted;
db<>fiddle demo
It may be useful in scenarios when there is need to "hide" a column for a time being before it could be safely dropped(like reworking corresponding application/reports etc.).
Related
I have the following constraint in CREATE statement:
UNIQUE (`field_name`)
These commands to remove constraint work in MySQL but not in H2:
DROP INDEX `field_name` ON `table_name`;
ALTER TABLE `table_name` DROP INDEX `field_name`;
I need a command which would work both in MySQL and H2 (MySQL is used in real environment and H2 in Unit tests)
Found the following workaround: removing column removes the constraint, so:
ALTER TABLE `table_name` ADD COLUMN `tmp_code` VARCHAR(50) NOT NULL DEFAULT 'TMP';
UPDATE `table_name`
SET `tmp_code` = `field_name`;
ALTER TABLE `table_name` DROP COLUMN `field_name`;
ALTER TABLE `table_name` ADD COLUMN `field_name` VARCHAR(50) NOT NULL;
UPDATE `table_name`
SET `field_name` = `tmp_code`;
ALTER TABLE `table_name` DROP COLUMN `tmp_code`;
Do SHOW CREATE TABLE to see the name of the UNIQUE KEY. Then you can proceed with the DROP or ALTER. It will probably say
UNIQUE KEY `col` (`col`),
The first col is the key name.
If you need to maintain the INDEX but get rid of the UNIQUEness constraint, then drop the key, then add a non-unique key.
I have a big MySQL InnoDB table having 5 million rows. I need to add a column to the table which will have a default int value.
What is the best way to do it? The normal alter table command appears to take a lot of time. Is there any better way to do it? Basically I want to know if there is any faster way or efficient way of doing it.
And if the table has foreign key references, is there any way other than alter table to do this?
Any help appreciated.
I would not say this is a better way, but ... You could create a separate table for the new data and set it up as foreign key relationship to the existing table. That would be "fast", but if the data really belongs in the main table and every (or most) existing records will have a value, then you should just alter the table and add it.
Suppose the table looked like this:
CREATE TABLE mytable
(
id INT NOT NULL AUTO_INCREMENT,
name VARCHAR(25),
PRIMARY KEY (id),
KEY name (name)
);
and you want to add an age column with
ALTER TABLE mytable ADD COLUMN age INT NOT NULL DEFAULT 0;
You could perform the ALTER TABLE in stages as follows:
CREATE TABLE mytablenew LIKE mytable;
ALTER TABLE mytablenew ADD COLUMN age INT NOT NULL DEFAULT 0;
INSERT INTO mytablenew SELECT id,name FROM mytable;
ALTER TABLE mytable RENAME mytableold;
ALTER TABLE mytablenew RENAME mytable;
DROP TABLE mytableold;
If mytable uses the MyISAM Storage Engine and has nonunique indexes, add two more lines
CREATE TABLE mytablenew LIKE mytable;
ALTER TABLE mytablenew ADD COLUMN address VARCHAR(50);
ALTER TABLE mytablenew DISABLE KEYS;
INSERT INTO mytablenew SELECT id,name FROM mytable;
ALTER TABLE mytable RENAME mytableold;
ALTER TABLE mytablenew RENAME mytable;
DROP TABLE mytableold;
ALTER TABLE mytable ENABLE KEYS;
This will let you see how many seconds each stage takes. From here, you can decide whether or not a straightforward ALTER TABLE is better.
This technique gets a little gory if there are foreign key references.
Your steps would be
SET UNIQUE_CHECKS = 0;
SET FOREIGN_KEY_CHECKS = 0;
Drop the foreign key references in mytable.
Perform the ALTER TABLE in Stages
Create the foreign key references in mytable.
SET UNIQUE_CHECKS = 1;
SET FOREIGN_KEY_CHECKS = 1;
Give it a Try !!!
A non-unique attribute of a table can be made unique by the query:
ALTER TABLE mytbl ADD UNIQUE (columnName);
I need to set a already unique attribute of a table NON-unique. Can anyone help me with the query?
alter table mytbl drop index columnName;
Use the above command for the same.
Drop it, like so:
ALTER TABLE yourTable DROP INDEX ItsName;
SQL Fiddle Demo
you can drop the unique index with the following statement:
ALTER TABLE mytbl DROP INDEX columnName
First of all you want to get name of index, you can do that by SHOW INDEX IN mytbl, then you can just ALTER TABLE:
ALTER TABLE mytbl DROP INDEX auto_index_name;
How do I add auto_increment to an existing column of a MySQL table?
I think you want to MODIFY the column as described for the ALTER TABLE command. It might be something like this:
ALTER TABLE users MODIFY id INTEGER NOT NULL AUTO_INCREMENT;
Before running above ensure that id column has a Primary index.
Method to add AUTO_INCREMENT to a table with data while avoiding “Duplicate entry” error:
Make a copy of the table with the data using INSERT SELECT:
CREATE TABLE backupTable LIKE originalTable;
INSERT backupTable SELECT * FROM originalTable;
Delete data from originalTable (to remove duplicate entries):
TRUNCATE TABLE originalTable;
To add AUTO_INCREMENT and PRIMARY KEY
ALTER TABLE originalTable ADD id INT PRIMARY KEY AUTO_INCREMENT;
Copy data back to originalTable (do not include the newly created column (id), since it will be automatically populated)
INSERT originalTable (col1, col2, col3)
SELECT col1, col2,col3
FROM backupTable;
Delete backupTable:
DROP TABLE backupTable;
More on the duplication of tables using CREATE LIKE:
Duplicating a MySQL table, indices, and data
Alter table table_name modify column_name datatype(length) AUTO_INCREMENT PRIMARY KEY
You should add primary key to auto increment, otherwise you got error in mysql.
Simply just add auto_increment Constraint In column or MODIFY COLUMN :-
ALTER TABLE `emp` MODIFY COLUMN `id` INT NOT NULL UNIQUE AUTO_INCREMENT FIRST;
Or add a column first then change column as -
1. Alter TABLE `emp` ADD COLUMN `id`;
2. ALTER TABLE `emp` CHANGE COLUMN `id` `Emp_id` INT NOT NULL UNIQUE AUTO_INCREMENT FIRST;
This worked for me in case you want to change the AUTO_INCREMENT-attribute for a not-empty-table:
1.)Exported the whole table as .sql file
2.)Deleted the table after export
2.)Did needed change in CREATE_TABLE command
3.)Executed the CREATE_TABLE and INSERT_INTO commands from the .sql-file
...et viola
I managed to do this with the following code:
ALTER TABLE `table_name`
CHANGE COLUMN `colum_name` `colum_name` INT(11) NOT NULL AUTO_INCREMENT FIRST;
This is the only way I could make a column auto increment.
INT(11) shows that the maximum int length is 11, you can skip it if you want.
Alter table table_name modify table_name.column_name data_type AUTO_INCREMENT;
eg:
Alter table avion modify avion.av int AUTO_INCREMENT;
if you have FK constraints and you don't want to remove the constraint from the table. use "index" instead of primary. then you will be able to alter it's type to auto increment
I had existing data in the first column and they were 0's.
First I made the first column nullable.
Then I set the data for the column to null.
Then I set the column as an index.
Then I made it a primary key with auto incrementing turned on. This is where I used another persons answer above:
ALTER TABLE `table_name` CHANGE COLUMN `colum_name` `colum_name` INT(11) NOT NULL AUTO_INCREMENT FIRST;
This Added numbers to all the rows of this table starting at one. If I ran the above code first it wasn't working because all the values were 0's. And making it an index was also required before making it auto incrementing.
Next I made the column a primary key.
This worked in my case , if you want to change the column attribute to auto-increment which is already having some data
1.GO to structure, select the column to want to change.
2.After selecting the column , choose primary key from the options below.
[1]: https://i.stack.imgur.com/r7w8f.png
3.Then change the column attribute to auto-increment using alter method
This is to alter the column adding PRIMARY key:
ALTER TABLE `schema_name`.`table_name`
CHANGE COLUMN `id` `id` INT(11) NOT NULL AUTO_INCREMENT ,
ADD UNIQUE INDEX `id_UNIQUE` (`id` ASC) VISIBLE,
ADD PRIMARY KEY (`id`);
I copied it from MySQL Workbench... I got curious to see if it was possible to do it all in one command. I'm a little rusty in SQL.
If you are working in an specific schema, you don't need to specify it.
The above statement will create the index, set the column as the PRIMARY KEY as well with just one query.
KEEP IN MIND: There could not be duplicated values in the same column, if there are, the statement will fail to commit.
ALTER TABLE Table name ADD column datatype AUTO_INCREMENT,ADD primary key(column);
I have a mySQL table called test:
create table test(
locationExpect varchar(120) NOT NULL;
);
I want to change the locationExpect column to:
create table test(
locationExpect varchar(120);
);
How can it be done quickly?
Do you mean altering the table after it has been created? If so you need to use alter table, in particular:
ALTER TABLE tablename MODIFY COLUMN new-column-definition
e.g.
ALTER TABLE test MODIFY COLUMN locationExpect VARCHAR(120);
Syntax to change column name in MySql:
alter table table_name change old_column_name new_column_name data_type(size);
Example:
alter table test change LowSal Low_Sal integer(4);
This should do it:
ALTER TABLE test MODIFY locationExpert VARCHAR(120)