- Can foreign key be null?
- What does drop table cascade constraints do?
- How do I drop a default constraint?
- Does drop table free up space Oracle?
- Which clause will allow you to delete a constraint?
- Can we drop a table with primary key?
- Is commit required after drop table in Oracle?
- What does drop table do in SQL?
- How can you remove a constraint?
- How do I drop a table with constraints in SQL?
- Does drop table drop constraints?
- How do you drop a unique constraint in SQL?
- What happens when you drop a table in Oracle?
- How do you drop a unique constraint?
- What is primary key constraint?
- What is a unique constraint violation?
- How do I see all constraints on a table in SQL?
- What are SQL constraints?
Can foreign key be null?
A foreign key containing null values cannot match the values of a parent key, since a parent key by definition can have no null values.
However, a null foreign key value is always valid, regardless of the value of any of its non-null parts.
A foreign key value is null if any part is null..
What does drop table cascade constraints do?
Second, specify CASCADE CONSTRAINTS clause to remove all referential integrity constraints which refer to primary and unique keys in the table. In case such referential integrity constraints exist and you don’t use this clause, Oracle returns an error and stops removing the table.
How do I drop a default constraint?
Syntax: ALTER TABLE
Does drop table free up space Oracle?
Unless you specify the PURGE clause, the DROP TABLE statement does not result in space being released back to the tablespace for use by other objects, and the space continues to count toward the user’s space quota. For an external table, this statement removes only the table metadata in the database.
Which clause will allow you to delete a constraint?
A constraint is similar to an index, although it can also be used to establish a relationship with another table. You use the CONSTRAINT clause in ALTER TABLE and CREATE TABLE statements to create or delete constraints.
Can we drop a table with primary key?
The syntax to drop a primary key in SQL is: ALTER TABLE table_name DROP PRIMARY KEY; table_name. The name of the table to modify.
Is commit required after drop table in Oracle?
CREATE TABLE and DROP TABLE statements do not commit a transaction if the TEMPORARY keyword is used. … SELECT causes an implicit commit before and after the statement is executed when you are creating nontemporary tables. (No commit occurs for CREATE TEMPORARY TABLE … SELECT .)
What does drop table do in SQL?
The drop table command is used to delete a table and all rows in the table. To delete an entire table including all of its rows, issue the drop table command followed by the tablename. drop table is different from deleting all of the records in the table.
How can you remove a constraint?
To delete a check constraintIn Object Explorer, expand the table with the check constraint.Expand Constraints.Right-click the constraint and click Delete.In the Delete Object dialog box, click OK.
How do I drop a table with constraints in SQL?
State’ — Drop the foreign key constraint by its name ALTER TABLE dbo….Select the tables you want to DROP.Select “Save to new query window”.Click on the Advanced button.Set Script DROP and CREATE to Script DROP.Set Script Foreign Keys to True.Click OK.Click Next -> Next -> Finish.View the script and then Execute.
Does drop table drop constraints?
DROP TABLE removes tables from the database. … DROP TABLE always removes any indexes, rules, triggers, and constraints that exist for the target table. However, to drop a table that is referenced by a view or a foreign-key constraint of another table, CASCADE must be specified.
How do you drop a unique constraint in SQL?
To delete a unique constraint using Table DesignerIn Object Explorer, right-click the table with the unique constraint, and click Design.On the Table Designer menu, click Indexes/Keys.In the Indexes/Keys dialog box, select the unique key in the Selected Primary/Unique Key and Index list.Click Delete.More items…•
What happens when you drop a table in Oracle?
Dropping a table removes the table definition from the data dictionary. … All synonyms for a dropped table remain, but return an error when used. All extents allocated for a table that is dropped are returned to the free space of the tablespace and can be used by any other object requiring new extents or new objects.
How do you drop a unique constraint?
Drop Unique Constraint The syntax for dropping a unique constraint in SQL Server is: ALTER TABLE table_name DROP CONSTRAINT constraint_name; table_name. The name of the table to modify.
What is primary key constraint?
The PRIMARY KEY constraint specifies that the constrained columns’ values must uniquely identify each row. Unlike other constraints which have very specific uses, the PRIMARY KEY constraint must be used for every table because it provides an intrinsic structure to the table’s data. … Tables can only have one primary key.
What is a unique constraint violation?
A unique constraint violation occurs when an UPDATE or INSERT statement attempts to insert a record with a key that already exists in the table. Take a look at the package that is throwing the error.
How do I see all constraints on a table in SQL?
1 AnswerSELECT * FROM user_cons_columns. WHERE table_name = ‘
What are SQL constraints?
SQL constraints are used to specify rules for the data in a table. Constraints are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the action is aborted.