An Essential Guide to SQLite CHECK Constraint

Summary: in this tutorial, you will learn how to use SQLite CHECK constraint to validate data before insert or update.

Introduction to SQLite CHECK constraints

SQLite CHECK constraints allow you to define expressions to test values whenever they are inserted into or updated within a column.

If the values do not meet the criteria defined by the expression, SQLite will issue a constraint violation and abort the statement.

The CHECK constraints allow you to define additional data integrity checks beyond UNIQUE or NOT NULL to suit your specific application.

SQLite allows you to define a CHECK constraint at the column level or the table level.

The following statement shows how to define a CHECK constraint at the column level:

CREATE TABLE table_name(
    ...,
    column_name data_type CHECK(expression),
    ...
);
Code language: SQL (Structured Query Language) (sql)

and the following statement illustrates how to define a CHECK constraint at the table level:

CREATE TABLE table_name(
    ...,
    CHECK(expression)
);
Code language: SQL (Structured Query Language) (sql)

In this syntax, whenever a row is inserted into a table or an existing row is updated, the expression associated with each CHECK constraint is evaluated and returned a numeric value 0 or 1.

If the result is zero, then a constraint violation occurred. If the result is a non-zero value or NULL, it means no constraint violation occurred.

Note that the expression of a CHECK constraint cannot contain a subquery.

SQLite CHECK constraint examples

Let’s take some examples of using the CHECK constraints.

1) Using SQLite CHECK constraint at the column level example

The following statement creates a new table named contacts:

CREATE TABLE contacts (
    contact_id INTEGER PRIMARY KEY,
    first_name TEXT    NOT NULL,
    last_name  TEXT    NOT NULL,
    email      TEXT,
    phone      TEXT    NOT NULL
                    CHECK (length(phone) >= 10) 
);
Code language: SQL (Structured Query Language) (sql)

In the contacts table, the phone column has a CHECK constraint:

CHECK (length(phone) >= 10) 
Code language: SQL (Structured Query Language) (sql)

This CHECK constraint ensures that the values in the phone column must be at least 10 characters.

If you attempt to execute the following statement, you will get a constraint violation error:

INSERT INTO contacts(first_name, last_name, phone)
VALUES('John','Doe','408123456');Code language: SQL (Structured Query Language) (sql)

Here is the error message:

Result: CHECK constraint failed: contacts
Code language: SQL (Structured Query Language) (sql)

The reason was that the phone number that you attempted to insert just has 9 characters while it requires at least 10 characters.

The following statement should work because the value in the phone column has 13 characters, which satisfies the expression in the CHECK constraint:

INSERT INTO contacts(first_name, last_name, phone)
VALUES('John','Doe','(408)-123-456');Code language: SQL (Structured Query Language) (sql)

2) Using SQLite CHECK constraints at the table level example

The following statement creates a new table named products:

CREATE TABLE products (
    product_id   INTEGER         PRIMARY KEY,
    product_name TEXT            NOT NULL,
    list_price   DECIMAL (10, 2) NOT NULL,
    discount     DECIMAL (10, 2) NOT NULL
                                DEFAULT 0,
    CHECK (list_price >= discount AND 
        discount >= 0 AND 
        list_price >= 0) 
);
Code language: SQL (Structured Query Language) (sql)

In this example, the CHECK constraint is defined at the table level:

CHECK (list_price >= discount AND 
            discount >= 0 AND 
            list_price >= 0) Code language: SQL (Structured Query Language) (sql)

The CHECK constraint ensures that list price is always greater or equal to discount and both discount and list price are greater or equal to zero.

The following statement violates the CHECK constraint because the discount is higher than the list price.

INSERT INTO products(product_name, list_price, discount)
VALUES('New Product',900,1000);    Code language: SQL (Structured Query Language) (sql)

The following statement also violates the CHECK constraint because the discount is negative:

INSERT INTO products(product_name, list_price, discount)
VALUES('New XFactor',1000,-10);    Code language: SQL (Structured Query Language) (sql)

Adding CHECK constraints to an existing table

As of version 3.25.2, SQLite does not support adding a CHECK constraint to an existing table.

However, you can follow these steps:

First, create a new table whose structure is the same as the table that you want to add a CHECK constraint. The new table should also include the CHECK constraint:

CREATE TABLE new_table (
    [...],
    CHECK ([...])
);
Code language: SQL (Structured Query Language) (sql)

To get the structure of the old table, you can use the .schema command. Check out the SQLite DESCRIBE table tutorial for more information.

Second, copy data from the old table to the new table.

INSERT INTO new_table SELECT * FROM old_table;Code language: SQL (Structured Query Language) (sql)

Third, drop the old table:

DROP TABLE old_table;Code language: SQL (Structured Query Language) (sql)

Fourth, rename the new table to the old one:

ALTER TABLE new_table RENAME TO old_table;Code language: SQL (Structured Query Language) (sql)

To make all statements above transaction-safe, you should execute all of them within a transaction like this:

BEGIN;
-- create a new table 
CREATE TABLE new_table (
    [...],
    CHECK ([...])
);
-- copy data from old table to the new one
INSERT INTO new_table SELECT * FROM old_table;

-- drop the old table
DROP TABLE old_table;

-- rename new table to the old one
ALTER TABLE new_table RENAME TO old_table;

-- commit changes
COMMIT;
Code language: SQL (Structured Query Language) (sql)

In this tutorial, you have learned how to use the SQLite CHECK constraint to ensure values in a column or a group of columns satisfies a condition defined by an expression.

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