I created a table but forgot to select primary and foreign key to join the table.
How can I fix this process?
You can use the ALTER TABLE Statement
ALTER TABLE table_name
ADD CONSTRAINT MyPrimaryKey
PRIMARY KEY (column1, column2...);
ALTER TABLE table_name
ADD CONSTRAINT MyForeignKey
FOREIGN KEY (column1) REFERENCES Table(column);
Related
I am new to sql and have not have much experience in it
Trying to create a relationship between two tables using a constraint.
adding a unique constraint and relationship for force the userid and consumerid between table1 and table2
any sample will help me
Thanks
You can use this FOREIGN KEY (blabla) REFERENCES secondTable(blabla) but only if the table is not created yet. Write it inside your CREATE TABLE query.
But if it already exists you can add it using ALTER TABLE tablename ADD CONSTRAINT `FK_myKey` FOREIGN KEY (`blabla`) REFERENCES `secondTable` (`blabla`) ON DELETE CASCADE ON UPDATE CASCADE;
I'm having the following problem with DB2:
I need to change a column name (column "A"), but the thing is that column A has a PK constraint and a FK constraint. So I need to drop this constraints first, change the column name and then create the constraints again. But I remember a Professor told me once that you can't create a foreign key in a column which already has values. Is that true?
This is my script:
ALTER TABLE TARGET_TABLE
DROP PRIMARY KEY PK_A CASCADE;
ALTER TABLE TARGET_TABLE
RENAME COLUMN A TO B;
alter table TARGET_TABLE
add CONSTRAINT PK_B PRIMARY KEY( B);
alter table TARGET_TABLE add CONSTRAINT FK_B FOREIGN KEY( B) REFERENCES OTHER_TABLE(C);
Thanks in advance.
You can create a foreign key on a column which already has values. The only restriction is that the values must be be valid for the FK you are defining.
If not you will get an error such as
SQL0667N The FOREIGN KEY "I..." cannot be created because the table contains
rows with foreign key values that cannot be found in the parent key of the
parent table. SQLSTATE=23520
I am not clear about what happens when a "foreign key constraint" is deleted specifying the option CASCADE.
For instance, consider this command
ALTER TABLE table1 DROP CONSTRAINT foreignKeyToTable2 CASCADE.
What the option CASCADE is supposed to do in this case? What would happen if I omitted it? And if I wrote RESTRICT instead of CASCADE?
Note: this example of query is excerpted from "Ramez Elmasri, Shamkant B. Navathe - Fundamentals of database systems, end of chapter 5".
The cascade option to drop a constraint is only needed when dropping primary keys, not when dropping a foreign key.
Consider this example in Postgres:
create table t1 (id integer, constraint pk_one primary key (id));
create table t2 (id integer primary key, id1 integer references t1);
When you try to run:
alter table t1 drop constraint pk_one;
You get:
ERROR: cannot drop constraint pk_one on table t1 because other objects depend on it
Detail: constraint t2_id1_fkey on table t2 depends on index pk_one
Hint: Use DROP ... CASCADE to drop the dependent objects too.
If you run:
alter table t1 drop constraint pk_one cascade;
you get:
NOTICE: drop cascades to constraint t2_id1_fkey on table t2
Telling you that the foreign key that needed the primary key was dropped as well.
Note that not all DBMS support a cascading drop. Postgres and Oracle do.
MySQL, SQL Server or Firebird do not. You need to drop the foreign keys manually in those DBMS.
I am trying to add a new column that will be a foreign key. I have been able to add the column and the foreign key constraint using two separate ALTER TABLE commands:
ALTER TABLE one
ADD two_id integer;
ALTER TABLE one
ADD FOREIGN KEY (two_id) REFERENCES two(id);
Is there a way to do this with one ALTER TABLE command instead of two? I could not come up with anything that works.
As so often with SQL-related question, it depends on the DBMS. Some DBMS allow you to combine ALTER TABLE operations separated by commas. For example...
Informix syntax:
ALTER TABLE one
ADD two_id INTEGER,
ADD CONSTRAINT FOREIGN KEY(two_id) REFERENCES two(id);
The syntax for IBM DB2 LUW is similar, repeating the keyword ADD but (if I read the diagram correctly) not requiring a comma to separate the added items.
Microsoft SQL Server syntax:
ALTER TABLE one
ADD two_id INTEGER,
FOREIGN KEY(two_id) REFERENCES two(id);
Some others do not allow you to combine ALTER TABLE operations like that. Standard SQL only allows a single operation in the ALTER TABLE statement, so in Standard SQL, it has to be done in two steps.
In MS-SQLServer:
ALTER TABLE one
ADD two_id integer CONSTRAINT fk FOREIGN KEY (two_id) REFERENCES two(id)
In MS SQL SERVER:
With user defined foreign key name
ALTER TABLE tableName
ADD columnName dataType,
CONSTRAINT fkName FOREIGN KEY(fkColumnName)
REFERENCES pkTableName(pkTableColumnName);
Without user defined foreign key name
ALTER TABLE tableName
ADD columnName dataType,
FOREIGN KEY(fkColumnName) REFERENCES pkTableName(pkTableColumnName);
For SQL Server it should be something like
ALTER TABLE one
ADD two_id integer constraint fk foreign key references two(id)
In Oracle :
ALTER TABLE one ADD two_id INTEGER CONSTRAINT Fk_two_id REFERENCES two(id);
2020 Update
It's pretty old question but people are still returning to it I see. In case the above answers did not help you, make sure that you are using same data type for the new column as the id of the other table.
In my case, I was using Laravel and I use "unsigned integer" for all of my ids as there is no point of having negative id LOL.
So for that, the raw SQL query will change like this:
ALTER TABLE `table_name`
ADD `column_name` INTEGER UNSIGNED,
ADD CONSTRAINT constrain_name FOREIGN KEY(column_name) REFERENCES foreign_table_name(id);
I hope it helps
PostgreSQL DLL to add an FK column:
ALTER TABLE one
ADD two_id INTEGER REFERENCES two;
For DB2, the syntax is:
ALTER TABLE one ADD two_id INTEGER FOREIGN KEY (two_id) REFERENCES two (id);
ALTER TABLE TableName
ADD NewColumnName INTEGER,
FOREIGN KEY(NewColumnName) REFERENCES [ForeignKey_TableName](Foreign_Key_Column)
You can do it like below in SQL Server
ALTER TABLE one
ADD two_id int foreign key
REFERENCES two(id)
If you also need to add default values in case you already have some rows in the table then add DEFAULT val
ALTER TABLE one
ADD two_id int DEFAULT 123,
FOREIGN KEY(two_id) REFERENCES two(id);
Try this:
ALTER TABLE product
ADD FOREIGN KEY (product_ID) REFERENCES product(product_ID);
For SQL Server with constraint name
ALTER TABLE one
ADD two_id INT NOT NULL,
CONSTRAINT FK_name FOREIGN KEY (two_id) REFERENCES two(id);
I'm changing constraints in my database and I need to drop some of them. I know that for a single constraint, the command is following:
ALTER TABLE tblApplication DROP CONSTRAINT constraint1_name;
However, when I try
ALTER TABLE tblApplication DROP (
CONSTRAINT constraint1_name,
CONSTRAINT constraint2_name,
CONSTRAINT constraint3_name
);
it doesn't work and I need to do:
ALTER TABLE tblApplication DROP CONSTRAINT constraint1_name;
ALTER TABLE tblApplication DROP CONSTRAINT constraint2_name;
ALTER TABLE tblApplication DROP CONSTRAINT constraint3_name;
Is there a way to remove more than one constraint in a single command? I'd like to avoid repeating ALTER TABLE tblApplication, just like with the ADD command:
ALTER TABLE tblApplication ADD (
CONSTRAINT contraint1_name FOREIGN KEY ... ENABLE,
CONSTRAINT contraint2_name FOREIGN KEY ... ENABLE,
CONSTRAINT contraint3_name FOREIGN KEY ... ENABLE
);
Yes you can. You just need to repeat 'drop constraint' per constraint. e.g.
alter table t1
drop constraint fk1
drop constraint fk2
/
Edit: I tested this against Oracle 11, and it worked fine. Don't know about older versions.
There is an alternative form to drop constraints related to a column in a table, also dropping the column with CASCADE:
ALTER TABLE table1 DROP (columnName) CASCADE CONSTRAINTS;
It is tested on Oracle 11g
example: we can drop constraints in MySQL by creating constraints to the variables like this way.
create table sample(id int, name varchar(30), marks int, constraint uid unique(id), constraint un unique(name));
alter table sample drop constraint uid, drop constraint un;
Yes, we can drop multiple at once:
ALTER TABLE TABLE NAME
DROP CONSTRAINTS CONSTRAINT VALUE
DROP CONSTRAINTS CONSTRAINT VALUE;