![]() The constraint can be applied for a single column or a group of columns. This constraint defines a business rule on a column. Location char(10) CONSTRAINT loc_un UNIQUE UNIQUE(column_name)įor Example: To create an employee table with Unique key, the query would be like, Syntax to define a Unique key at table level: Syntax to define a Unique key at column level: A column(s) can have a null value but the values cannot be duplicated. This constraint ensures that a column or a group of columns in each row have a distinct value. Which means a null value is not allowed.įor Example: To create a employee table with Null value, the query would be like This constraint ensures all rows in the table contain a definite value for the column which is specified as not null. Mgr_id number(5) REFERENCES employee(id), Product_id number(5) CONSTRAINT pd_id_fk REFERENCES, product(product_id),ĬONSTRAINT od_id_pk PRIMARY KEY(order_id),ĬONSTRAINT pd_id_fk FOREIGN KEY(product_id) REFERENCES product(product_id)Ģ) If the employee table has a 'mgr_id' i.e, manager id as a foreign key which references primary key 'id' within the same table, the query would be like, ( order_id number(5) CONSTRAINT od_id_pk PRIMARY KEY, ( product_id number(5) CONSTRAINT pd_id_pk PRIMARY KEY, FOREIGN KEY(column_name) REFERENCES referenced_table_name(column_name) ġ) Lets use the "product" table and "order_items". Syntax to define a Foreign key at table level: REFERENCES Referenced_Table_name(column_name) Syntax to define a Foreign key at column level: ![]() One or more columns can be defined as Foreign key. For a column to be defined as a Foreign Key, it should be a defined as a Primary Key in the table which it is referring. It establishes a relationship between two columns in the same table or between different tables. This constraint identifies any column referencing the PRIMARY KEY in another table. ( id number(5) CONSTRAINT emp_id_pk PRIMARY KEY,ĪLTER TABLE employee ADD CONSTRAINT PK_EMPLOYEE_ID PRIMARY KEY (id)Ģ) SQL Foreign key or Referential Integrity : is optional.įor Example: To create an employee table with Primary Key constraint, the query would be like. For syntax, check ALTER TABLE and CREATE TABLE. ![]() UNIQUE constraint is only supported when NOT ENFORCED is used. column_name1, column_name2 are the names of the columns which define the primary Key. Dedicated SQL pool supports these table constraints: PRIMARY KEY is only supported when NONCLUSTERED and NOT ENFORCED are both used.PRIMARY KEY (column_name1,column_name2.) Syntax to define a Primary key at table level: Syntax to define a Primary key at column level:Ĭolumn name datatype PRIMARY KEY This constraint defines a column or combination of columns which uniquely identifies each row in the table. This is called column-level definition.Ģ) The constraints can be specified after all the columns are defined. The constraints available in SQL are Foreign Key, Not Null, Unique, Check.ġ) The constraints can be specified immediately after the column definition. Integrity Constraints are used to apply business rules for the database tables.
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