Here in the above, the first two rows have been inserted, but in the 3rd line, we forcefully tried to enter the value of com_id COM1, which already exists and this is violating the uniqueness of the PRIMARY KEY and the above error message appeared. Sqlite> INSERT INTO company VALUES("COM2","K M P CORP.") Įrror: UNIQUE constraint failed: _id Now, sqlite> INSERT INTO company VALUES("COM1","T S LTD.") Here we insert a row and value against column com_id is NULL, and it has been accepted though the com_id is a PRIMARY KEY because SQLite allows NULL values in a PRIMARY KEY column. sqlite> INSERT INTO company VALUES(NULL,"T S LTD.") The PRAGMA command can be used to see the structure of the table. Here in the above example com_id is the single column PRIMARY KEY. According to the SQL standard, PRIMARY KEY should always imply NOT NULL.
The PRIMARY KEY is optional for ordinary tables. More than one PRIMARY KEY clause is not possible in a CREATE TABLE statement. For the purposes of determining the uniqueness of primary key values, NULL values are considered distinct from all other values, including other NULLs. Each row in a table with a primary key must have a unique combination of values in its primary key columns. if a PRIMARY KEY clause is specified as a table-constraint, then the primary key of the table consists of the list of columns specified as part of the PRIMARY KEY clause. If the keywords PRIMARY KEY are added to a column definition, then the primary key for the table consists of that single column. Sqlite> INSERT INTO company VALUES("COM1",NULL) Įrror: NOT NULL constraint failed: _nameīut, here in the above we forcefully tried to enter the value of second column NULL a violation of constraint occurred. Here also no problem arise at the time of insertion, though the value of the second column is blank. Sqlite> INSERT INTO company VALUES("COM1","") No problem arise at the time of insertion. Sqlite> INSERT INTO company VALUES("COM1","T S LTD.") Now look at the usage of the NOT NULL with CREATE TABLE statement. Attempting to set the column value to NULL when inserting a new row or updating an existing one causes a constraint violation. Not surprisingly, a NOT NULL constraint dictates that the associated column may not contain a NULL value.
The following column constraints that should be enforced when data is inserted:Ī NOT NULL constraint may only be attached to a column definition, not specified as a table constraint. The column level constraints can be applied only on a specific column where as table level constraints can be applied to the whole table. We can use two types of constraints, that is column level or table level constraint. The CONSTRAINTS are an integrity which defines some conditions that restrict the column to contain the true data while inserting or updating or deleting.
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