The row won’t be inserted, and you will get an error saying that: FOREIGN KEY constraint failed.I just specced into arcane from int/quality before the patch and it's WAY better. Then try to insert a new student with a departmentId that doesn’t exist in the departments’ table: INSERT INTO Students(StudentName,DepartmentId) VALUES('John', 5) The two statements should insert two departments into departments table, you can ensure that the two values were inserted by running the query “SELECT * FROM Departments” after that: INSERT INTO Departments VALUES(2, 'Arts')
Let’s insert two departments “IT” and “Arts” into the departments table as following: INSERT INTO Departments VALUES(1, 'IT') If you are tried to insert a departmentId value that doesn’t exist in the departments table, the foreign key constraint would prevent you to do that. In this example, the Departments table has a Foreign key constraint to the Students table, so any departmentId value inserted in the students table must exist in the departments table. To check how foreign key constraints can prevent undefined element or value to be inserted in a table that has a relation to another table, we will look into the following example.
INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,įOREIGN KEY(DepartmentId) REFERENCES Departments(DepartmentId) INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, So, if we created a foreign key constraint on the DepartmentId on the Students table, each inserted departmentId have to present in the Departments table. Now, we will see how does the foreign key constraint can be helpful to ensure that the value of the department id in the students table must exist in the departments table. Each student belongs to a department i.e., each student has a departmentId column. The Students table have a list of students, and the departments table has a list of the departments. Suppose if we have two tables Students and Departments. Note that Foreign keys constraints are not enabled by default in SQLite, you have to enable them first by the running the following command: PRAGMA foreign_keys = ON įoreign key constraints were introduced in SQLite starting from version 3.6.19. In this case, when you try to insert a value on that column, then the foreign key will ensure that the inserted value exists in the table’s column.
And if you want to ensure that the value inserted in one of them must exist in the other table’s column, then you should use a “Foreign key Constraint” on the column in common. While working with multiple tables, when there are two tables that relate to each other with one column in common. The SQLite foreign key is a constraint that verifies the existence of value present in one table to another table that has a relation with the first table where the foreign key is defined. You can’t insert a value less than 10 in the “Quantity” column. Quantity INTEGER NOT NULL CHECK(Quantity > 10) SQLite check constraint a condition to check an inserted value, if the value doesn’t match the condition, it won’t be inserted. Note that, this applies on the values of the column “EmployeeId” only. This will enforce the “EmployeeId” value to be unique, no duplicated values will be allowed. SQLite Unique constraint it will prevent duplicate values among all the values of the column.įor example: EmployeeId INTEGER NOT NULL UNIQUE If you write an insert statement, and you didn’t specify any value for that column, the column will have the value 0. SQLite Default constraint if you don’t insert any value in a column, the default value will be inserted instead.įor Example: ColumnName INTEGER DEFAULT 0 SQLite Not null constraint prevents a column from having a null value:ĬolumnName INTEGER NOT NULL DEFAULT Constraint To create a combination of columns as a primary key:.There are a lot of different ways to define a primary key on a table like: The primary key can be applied to only one column or on a combination of columns, in the latter case, the combination of the columns’ values should be unique for all the tables’ rows. SQLite Primary KeyĪll the values on a primary key column should be unique and not null Columns constraints are defined when creating a table, in the column definition. Column constraints enforce constraints and rules to the values inserted on a column in order to validate the inserted data.
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