In mysql, when creating a table, you can append:
unique(username)
but i dont know how to alter a table to add that containt.
I was trying:
alter table Company constraint unique (username)
I mixed up the syntax.
alter table Company add constraint unique (username);
but this will fail if there are duplicates... So either drop and recreate if it is a sample db, or delete out duplicates.
Related
I am new to SQL and I want to learn it.
I have created a table to practice 'adding' and 'removing' primary key using constraints in MySQL.
syntax: create table test1 (id_no int,name varchar(25));
now, i wanted to add a primarykey to the existing table, and i had used a constraint so that i could remove primary key from column in future.
syntax: alter table test1 add constraint pk_id primary key(id_no);
when i tried to drop constraint
syntax: alter table test1 drop constraint pk_id;
it says:
ERROR 3940 (HY000): Constraint 'pk_id' does not exist.
how to add constraint to my id_no column.
i had tried different methods it still says constraint does not exist
please help me
You can use this in MySQL to drop the primary key.
ALTER TABLE test1 DROP PRIMARY KEY;
Try this instead:
alter table 'test1' drop constraint Primary key pk_id;
I have following problem:
the table "tblverleihdaten" is supposed to implement or reference the K_Nr column from the table "tblkunden" I've already created both tables but now I don't know how I can alter them to refrence each other like in the picture. I've been searching online but couldn't transfer any of the solutions on my problem.
simp,y alter the table tbverleihdaten with these statements
ALTER TABLE tblverleihdaten
ADD FOREIGN KEY (K_Nr) REFERENCES tblkunden(K_Nr);
ALTER TABLE tblverleihdaten
ADD FOREIGN KEY (L_Nr) REFERENCES tbllaptops(L_Nr);
For more information
But then you only can add rows to tbverleihdaten if a corresponding Id exists in both other tables
On PHPMyAdmin, I would like to DROP a FOREIGN KEY with
ALTER TABLE information DROP FOREIGN KEY IDX_29791883B30676A7
Because when I do the next query it's not working:
ALTER TABLE information DROP INDEX IDX_29791883B30676A7
Cannot drop index 'IDX_29791883B30676A7': needed in a foreign key constraint
However, the second query as the error that the index is use as foreigner key.
Fine, but when I do the first query I get this error:
Can't DROP 'IDX_29791883B30676A7'; check that column/key exists
So the questions are:
How do I acually check that,
How can I finally drop that key.
Foreign keys by default are prefixed with 'FK' not 'IDX'. You are trying to remove an index instead of a foreign key. You mentioned that the foreign key is: FK_29791883B30676A7 so the correct way to remove it will be:
ALTER TABLE information DROP FOREIGN KEY FK_29791883B30676A7
Once a foreign key has been created, you may find that you wish to drop the foreign key from the table. You can do this with the ALTER TABLE statement in SQL Server
I have the same problem with foreign keys, well I found a solution to removing constraint. For first take a look at:
SHOW CREATE TABLE information;
You will found the name of the rule, and then just drop it:
ALTER TABLE information DROP CONSTRAINT `name_of_rule`;
Try this
SET FOREIGN_KEY_CHECKS=0;
ALTER TABLE information DROP FOREIGN KEY IDX_29791883B30676A7;
ALTER TABLE information DROP INDEX IDX_29791883B30676A7;
SET FOREIGN_KEY_CHECKS=1;
How can I drop the "Unique Key Constraint" on a column of a MySQL table using phpMyAdmin?
A unique constraint is also an index.
First use SHOW INDEX FROM tbl_name to find out the name of the index. The name of the index is stored in the column called key_name in the results of that query.
Then you can use DROP INDEX:
DROP INDEX index_name ON tbl_name
or the ALTER TABLE syntax:
ALTER TABLE tbl_name DROP INDEX index_name
You can DROP a unique constraint from a table using phpMyAdmin as requested as shown in the table below. A unique constraint has been placed on the Wingspan field. The name of the constraint is the same as the field name, in this instance.
The indexes capable of placing a unique key constraint on a table are PRIMARY and UNIQUE indexes.
To remove the unique key constraint on a column but keep the index, you could remove and recreate the index with type INDEX.
Note that it is a good idea for all tables to have an index marked PRIMARY.
To add UNIQUE constraint using phpmyadmin, go to the structure of that table and find below and click that,
To remove the UNIQUE constraint, same way, go to the structure and scroll down till Indexes Tab and find below and click drop,
Hope this works.
Enjoy ;)
If you want to remove unique constraints from MySQL database table, use alter table with drop index.
Example:
CREATE TABLE unique_constraints (
unid INT,
activity_name VARCHAR(100),
CONSTRAINT activty_uqniue UNIQUE (activity_name),
PRIMARY KEY (unid)
);
ALTER TABLE unique_constraints
DROP INDEX activty_uqniue;
Where activty_uqniue is UNIQUE constraint for activity_name column.
For WAMP 3.0 :
Click Structure
Below Add 1 Column you will see '- Indexes'
Click -Indexes and drop whichever index you want.
The constraint could be removed with syntax:
ALTER TABLE
As of MySQL 8.0.19, ALTER TABLE permits more general (and SQL standard) syntax for dropping and altering existing constraints of any type, where the constraint type is determined from the constraint name: ALTER TABLE tbl_name DROP CONSTRAINT symbol;
Example:
CREATE TABLE tab(id INT, CONSTRAINT unq_tab_id UNIQUE(id));
-- checking constraint name if autogenerated
SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS WHERE TABLE_NAME = 'tab';
-- dropping constraint
ALTER TABLE tab DROP CONSTRAINT unq_tab_id;
db<>fiddle demo
This might help:
Inside your sql terminal
FIRST STEP:
SHOW INDEX FROM {YOUR_TABLE_NAME}
SECOND STEP:
SHOW INDEX FROM {YOUR_TABLE_NAME} WHERE Column_name='ACTUAL_COLUMN_NAME_YOU_GOT_FROM_FIRST_STEP_OUTPUT'
THIRD STEP:
ORIGINAL_KEY_NAME_VALUE = SECOND_STEP_RESPONSE["Key_name"]
FOURTH STEP:
ALTER TABLE {YOUR_TABLE_NAME} DROP INDEX ${ORIGINAL_KEY_NAME_VALUE}
while dropping unique key we use index
ALTER TABLE tbl
DROP INDEX unique_address;
my table name is buyers which has a unique constraint column emp_id now iam going to drop the emp_id
step 1: exec sp_helpindex buyers, see the image file
step 2: copy the index address
step3: alter table buyers drop constraint [UQ__buyers__1299A860D9793F2E]
alter table buyers
drop column emp_id
note:
Blockquote
instead of buyers change it to your table name :)
Blockquote
thats all column name emp_id with constraints is dropped!
I have a table whose primary key is used in several other tables and has several foreign keys to other tables.
CREATE TABLE location (
locationID INT NOT NULL AUTO_INCREMENT PRIMARY KEY
...
) ENGINE = InnoDB;
CREATE TABLE assignment (
assignmentID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
locationID INT NOT NULL,
FOREIGN KEY locationIDX (locationID) REFERENCES location (locationID)
...
) ENGINE = InnoDB;
CREATE TABLE assignmentStuff (
...
assignmentID INT NOT NULL,
FOREIGN KEY assignmentIDX (assignmentID) REFERENCES assignment (assignmentID)
) ENGINE = InnoDB;
The problem is that when I'm trying to drop one of the foreign key columns (ie locationIDX) it gives me an error.
"ERROR 1025 (HY000): Error on rename"
How can I drop the column in the assignment table above without getting this error?
As explained here, seems the foreign key constraint has to be dropped by constraint name and not the index name.
The syntax is:
ALTER TABLE footable DROP FOREIGN KEY fooconstraint;
The foreign keys are there to ensure data integrity, so you can't drop a column as long as it's part of a foreign key. You need to drop the key first.
I would think the following query would do it:
ALTER TABLE assignmentStuff DROP FOREIGN KEY assignmentIDX;
As everyone said above, you can easily delete a FK. However, I just noticed that it can be necessary to drop the KEY itself at some point. If you have any error message to create another index like the last one, I mean with the same name, it would be useful dropping everything related to that index.
ALTER TABLE your_table_with_fk
drop FOREIGN KEY name_of_your_fk_from_show_create_table_command_result,
drop KEY the_same_name_as_above
Check what's the CONSTRAINT name and the FOREIGN KEY name:
SHOW CREATE TABLE table_name;
Remove both the CONSTRAINT name and the FOREIGN KEY name:
ALTER TABLE table_name
DROP FOREIGN KEY the_name_after_CONSTRAINT,
DROP KEY the_name_after_FOREIGN_KEY;
Hope this helps!
Hey I followed some sequence above,
and found some solution.
SHOW CREATE TABLE footable;
You will get FK Constrain Name like
ProjectsInfo_ibfk_1
Now you need to remove this constraints. by alter table commantd
alter table ProjectsInfo drop foreign key ProjectsInfo_ibfk_1;
Then drop the table column,
alter table ProjectsInfo drop column clientId;
Here's a way to drop foreign key constraint, it will work.
ALTER TABLE location.location_id
DROP FOREIGN KEY location_ibfk_1;
You usually get this error if your tables use the InnoDB engine. In that case you would have to drop the foreign key, and then do the alter table and drop the column.
But the tricky part is that you can't drop the foreign key using the column name, but instead you would have to find the name used to index it. To find that, issue the following select:
SHOW CREATE TABLE region;
This should show you a row ,at left upper corner click the +option ,the click the full text raio button then click the go .there you will get the name of the index, something like this:
CONSTRAINT region_ibfk_1 FOREIGN KEY (country_id) REFERENCES country (id) ON DELETE NO ACTION ON UPDATE NO ACTION
Now simply issue an:
alter table region drop foreign key region_ibfk_1;
or
more simply just type:-
alter table TableName drop foreign key TableName_ibfk_1;
remember the only thing is to add _ibfk_1 after your tablename to make like this:- TableName_ibfk_1
first need to get actual constrain name by this query
SHOW CREATE TABLE TABLE_NAME
This query will result constrain name of the foreign key, now below query will drop it.
ALTER TABLE TABLE_NAME DROP FOREIGN KEY COLUMN_NAME_ibfk_1
last number in above constrain name depends how many foreign keys you have in table
You can not drop the foreign key column because it is being referenced from the table assignmentStuff. So you should first drop the foreign key constraint assignmentStuff.assignmentIDX.
A similar question has already been asked here. Check also here for more info.
Try this:
alter table Documents drop
FK__Documents__Custo__2A4B4B5E
step1: show create table vendor_locations;
step2: ALTER TABLE vendor_locations drop foreign key vendor_locations_ibfk_1;
it worked for me.