SQL > ALTER TABLE > Drop Constraint Syntax.
DROP a UNIQUE Constraint.
An unique key is a key that can be a column or a group of column which defines uniqueness for that record/row in the table. To drop a UNIQUE constraint, use the following SQL: SQL Server / Oracle / MS Access: For example, in the stores_demo database, there is a primary-key constraint on the order_num column in the orders table. ... by disabling the keys (which turns off all UNIQUE and PRIMARY keys) and by disabling all FK checks, you have everything covered. Here, the AGE column is set to UNIQUE, so that you cannot have two records with the same age. Once CHECK CONSTRAINTS are enabled, I am sure there will be a way to turn them off temporarily. Add unique key in mysql Name of the table to create the unique constraint on: all: all: tablespace 'Tablespace' to create the index in. Since we can specify constraints on a table, there needs to be a way to remove this constraint as well. So my strategy is to drop the unique constraint, add a new column to the table, and then set the new constraints. In this snippet you will learn how to add and drop unique key in Mysql. Corresponds to file group in mssql: all: validate: Should be true if the unique constraint shall 'ENABLE VALIDATE' set, or false if the 'ENABLE NOVALIDATE' shall. For example, the following SQL query creates a new table called CUSTOMERS and adds five columns. Right now, you can create tables that have them but they are not enforced. ... To drop a UNIQUE constraint, use the following SQL query. FOREIGN KEY A FOREIGN KEY in MySQL creates a link between two tables by one specific column of both tables. The SQL syntax to remove a constraint from a table is,
Summary: in this tutorial, you will learn how to use the MySQL UNIQUE index to prevent duplicate values in one or more columns in a table.. Introduction to the MySQL UNIQUE index. I could easily do this as follows in the older version of MySQL (4.0.X) as follows:
A PRIMARY KEY constraint for a table enforces the table to accept unique data for a specific column and this constraint creates a unique index for accessing the table faster. Naming of UNIQUE Constraint: MySQL / SQL Server / MS Access / Oracle: CREATE TABLE STUDENTS( ROLL_NO INT NOT NULL, STU_NAME VARCHAR (35) NOT NULL, STU_AGE INT NOT NULL, STU_ADDRESS VARCHAR (35), CONSTRAINT stu_Info UNIQUE(STU_NAME, STU_ADDRESS), PRIMARY KEY (ROLL_NO) ); Set UNIQUE Constraint on a already created table Name Description Required for Supports Since; catalogName: Name of the catalog: all: 3.0: constraintName: Name of unique constraint to drop: all: all: schemaName: Name of the schema I have no primary key defined on the table. In SQL, this is done via the ALTER TABLE statement..
When you drop a primary-key constraint or a unique constraint that has a corresponding foreign key, any associated referential constraints are also dropped. Constraints can be placed on a table to limit the type of data that can go into a table. Unique key is different from primary key, unique key can be null whereas primary key cannot be null.