Which of these is not a valid constraint?
Explanation
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Theory
  • SQL UNIQUE Constraint

    The UNIQUE constraint uniquely identifies each record in a database table.
    The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
    A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
    Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table. Read more: SQL UNIQUE Constraint
  • CHECK Constraint

    The CHECK Constraint enables a condition to check the value being entered into a record. If the condition evaluates to false, the record violates the constraint and isn't entered into the table.

    Example

    The following SQL creates a new table called CUSTOMERS. Here, we add a CHECK with AGE column, so that you can not have any CUSTOMER below 18 years:
    CREATE TABLE CUSTOMERS(
           ID   INT              NOT NULL,
           NAME VARCHAR (20)     NOT NULL,
           AGE  INT              NOT NULL CHECK (AGE >= 18),  
           PRIMARY KEY (ID)
    );
    
    If CUSTOMERS table has already been created, then to add a CHECK constraint to AGE column, you would write a statement similar to the following:
    ALTER TABLE CUSTOMERS
       MODIFY AGE INT NOT NULL CHECK (AGE >= 18 );
    
    Naming the constraint in multiple columns as well:
    ALTER TABLE CUSTOMERS
       ADD CONSTRAINT myCheckConstraint CHECK(AGE >= 18);
    
  • NOT NULL Constraint

    By default, a column can hold NULL values. If you do not want a column to have a NULL value, then you need to define such constraint on this column specifying that NULL is now not allowed for that column.
    A NULL is not the same as no data, rather, it represents unknown data.

    Example

    The following SQL creates a new table called CUSTOMERS, two of which, ID and NAME, specify not to accept NULLs:
    CREATE TABLE CUSTOMERS(
           ID   INT              NOT NULL,
           NAME VARCHAR (20)     NOT NULL,
           ADDRESS  CHAR (25) ,
           PRIMARY KEY (ID)
    );
    
    If CUSTOMERS table has already been created, then to add a NOT NULL constraint to SALARY column in Oracle and MySQL:
    ALTER TABLE CUSTOMERS
       MODIFY SALARY  DECIMAL (18, 2) NOT NULL;
    
    Read more: NOT NULL Constraint
  • SQL PRIMARY KEY Constraint

    The PRIMARY KEY constraint uniquely identifies each record in a database table.
    Primary keys must contain UNIQUE values.
    A primary key column cannot contain NULL values.
    Most tables should have a primary key, and each table can have only ONE primary key.
    Read more: SQL UNIQUE Constraint

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