Check Constraint In MySQL 8.0

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Copyright © 2019, Oracle and/or its affiliates. All rights reserved.
PRAVEENKUMAR HULAKUND
Principal Engineer
MySQL Runtime Team, Oracle
23 August 2019
CHECK CONSTRAINT in MySQL 8.0

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2

Agenda
Check Constraint Introduction
Create Check Constraint
Check Constraint Evaluation
Alter Check Constraint
Drop Check Constraint
Check Constraint Expression
Information_schema Tables
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6
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Copyright © 2019, Oracle and/or its affiliates. All rights reserved. | 4
Check Constraint Introduction

Check Constraint, a type of integrity constraint

Requirement that must be met by each row.
●
A boolean expression.
●
Can refer a single or multiple columns.
●
Result of expression can be
TRUE / FALSE - for non-NULL column values.
UNKNOWN - for NULL column value.
●
Constraint Satisfied for Results - TRUE or UNKNOWN.
CREATE TABLE employees (
id INT PRIMARY KEY CHECK(id > 0),
name VARCHAR(255) NOT NULL,
hire_date DATE NOT NULL,
end_date DATE,
CHECK(end_date > hire_date));

Check Constraint Typical Scenarios

Check Constraint used to
●
Limit value range.
●
Allow only certain values.
●
Enforce relation between columns of a same row.
id INT PRIMARY KEY CHECK(id > 0),
age INT CHECK (age > 18 and age < 60),
end_date DATE,
country varchar(255),
CHECK (country == “INDIA” OR id > 50000),

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Create Check Constraint

id INT PRIMARY KEY CONSTRAINT
id_check CHECK(id > 0),
end_date DATE,
CONSTRAINT end_date_check
Create Check Constraint, with SQL standard syntax!

CREATE TABLE and ALTER TABLE supports following syntax in
column definition and table definition.
[ CONSTRAINT [symbol] ] CHECK ( condition) [ [ NOT ] ENFORCED ]
SHOW CREATE TABLE employeesG
....
Create Table: CREATE TABLE èmployees` (
ìd` int(11) NOT NULL,
`name` varchar(255) NOT NULL,
`hire_date` date NOT NULL,
ènd_date` date DEFAULT NULL,
PRIMARY KEY (ìd`),
CONSTRAINT ènd_date_check` CHECK
((ènd_date` > `hire_date`)),
CONSTRAINT ìd_check` CHECK ((ìd` > 0))
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
COLLATE=utf8mb4_0900_ai_ci


Name is generated if not specified.
Format: <table_name>_chk_<Ordinal_number>
Create Check Constraint - Con’t, Name is optional!
id_check CHECK(id > 0),
end_date DATE,
....
Create Table: CREATE TABLE èmployees` (
PRIMARY KEY (ìd`),
CONSTRAINT èmployees_chk_1` CHECK

Create Check Constraint - Con’t, ENFORCED by
default!

NOT ENFORCED clause is supported to create without enforcing
it.
id_check CHECK(id > 0) NOT ENFORCED,
name varchar(255) NOT NULL,
hire_date date NOT NULL,
end_date date DEFAULT NULL,
CHECK (end_date > hire_date));
Query OK, 0 rows affected (0.02 sec)
...
CREATE TABLE èmployees` (
PRIMARY KEY (ìd`),
CONSTRAINT ìd_check` CHECK ((ìd` >
0)) /*!80016 NOT ENFORCED */
1 row in set (0.01 sec)


ALTER TABLE supports following clause to add constraint to an
existing table.
ALTER TABLE <table_name>
ADD [CONSTRAINT [symbol]] CHECK (condition) [[NOT] ENFORCED]
Create Check Constraint - Con’t, in an existing table
id INT PRIMARY KEY,
end_date date);
ALTER TABLE employees
ADD CONSTRAINT id_check CHECK(id > 0),
ADD CHECK (end_date > hire_date);
....
PRIMARY KEY (`id`),


Evaluated for DML operations,
●
INSERT
●
UPDATE
●
REPLACE
●
LOAD DATA
●
LOAD XML

Evaluated for ALTER TABLE, CREATE TABLE ... SELECT operations.

Evaluated for each row.

Constraints in ENFORCED state are evaluated.

Constraint violation error is reported on ERROR.

With IGNORE clause in DML operations warning is reported and the offending row is
skipped.

mysql> INSERT INTO employees VALUES (1, "Rahul.D", "1996-06-20", "2011-07-16");
Query OK, 1 row affected (0.00 sec)
mysql> INSERT INTO employees VALUES (2, "Rahul.D", "1996-06-20", NULL);
ERROR 3819 (HY000): Check constraint 'employees_chk_1' is violated.
mysql> INSERT IGNORE INTO employees VALUES (3, "Rahul.D", "1996-06-20", "1995-07-16");
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> SHOW WARNINGS;
+---------+------+-------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------+
| Warning | 3819 | Check constraint 'employees_chk_1' is violated. |
+---------+------+-------------------------------------------------+
Check Constraint Evaluation - Cont’d

Alter Check Constraint


ALTER TABLE supports following clause to ALTER enforcement
state.
ALTER TABLE <table_name> ALTER CHECK symbol [ NOT ] ENFORCED
Alter Constraint enforcement state.
id_check CHECK(id > 0) NOT ENFORCED,
end_date date DEFAULT NULL,
CHECK (end_date > hire_date));
Query OK, 0 rows affected (0.02 sec)
ALTER CHECK id_check ENFORCED,
ALTER CHECK employees_chk_1 NOT ENFORCED;
...
PRIMARY KEY (`id`),
((`end_date` > `hire_date`)) /*!80016 NOT
ENFORCED */,


ALTER TABLE statement support following clause to drop check
constraint.
ALTER TABLE <table_name> DROP CHECK symbol;
DROP CHECK id_check,
DROP CHECK employees_chk_1;
...
Create Table: CREATE TABLE `employees`
(
PRIMARY KEY (`id`)


Table of permitted and non-permitted constructs.

Check Constraint Expression is syntatically valid always.
●
RENAME or DROP of column used constraint is not allowed.

Implicit type conversion occurs if operand type mismatches.
ss
Generated and Non-generated columns.
Literals, operators and deterministic
built-in functions
(ABS(), LENGTH(), TIMEDIFF(), ISNULL)
Columns with AUTO_INCREMENT attribute.
Non-deterministic built-in functions
(RAND, LAST_INSERT_ID, AVG)
Sub-query
Environment variables (CURRENT_USER,
CURRENT_DATE).
System, user-defined and stored program
variables.
Stored function and user defined functions.

INFORMATION_SCHEMA Tables


New Table INFORMATION_SCHEMA.CHECK_CONSTRAINTS:

INFORMATION_SCHEMA.TABLE_CONSTRAINTS:
INFORMATION_SCHEMA Tables

MySQL documentation on check constraints:
https://dev.mysql.com/doc/refman/8.0/en/create-table-check-constraints.html
Blogpost on check constraint:
https://mysqlserverteam.com/mysql-8-0-16-introducing-check-constraint/
MySQL documentation on CREATE TABLE and ALTER TABLE syntax:
https://dev.mysql.com/doc/refman/8.0/en/create-table.html
https://dev.mysql.com/doc/refman/8.0/en/alter-table.html
MySQL documentation on INFORMATION_SCHEMA tables:
https://dev.mysql.com/doc/refman/8.0/en/check-constraints-table.html
https://dev.mysql.com/doc/refman/8.0/en/table-constraints-table.html
MySQL documentation on SHOW table:
https://dev.mysql.com/doc/refman/8.0/en/show-create-table.html
References

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