About types of column constraints

This topic describes how to use different types of constraints in OceanBase Database in MySQL mode.

Prerequisites

The current user has the CREATE and ALTER privileges on the target table.

Constraint types

To ensure that the data in the table conforms to the business rules, you can define constraints at the column level.

The constraints limit the type of values that can be stored in a column. If you attempt to enter or update a value that does not satisfy the constraint on the corresponding column, an error is returned and the operation is rolled back. Likewise, if you attempt to add a constraint that conflicts with the existing data to a column in an existing table, an error is returned and the operation is rolled back.

The types of constraints are described as follows:

• NOT NULL constraint: enforces that the constrained column must not contain null values.

• UNIQUE constraint: enforces that the constrained column must not contain duplicate values, but it can contain multiple null values.

• PRIMARY KEY constraint: It is the combination of the NOT NULL constraint and the UNIQUE constraint.

• FOREIGN KEY constraint: enforces that the constrained column must contain only values from the primary key column of another table.

  Note

  By default, the FOREIGN KEY constraint is enabled. You can set the foreign_key_checks variable to disable this constraint.

Example 1

• The NOT NULL constraint on the w_name column in the ware table requires that each warehouse must have a name.

  obclient> CREATE TABLE ware(
              id int,
              w_name varchar(256) not null,
              PRIMARY KEY (id)
           ) ;
  Query OK, 0 rows affected
  
  obclient> DESC ware;
  +--------+--------------+------+-----+---------+-------+
  | Field  | Type         | Null | Key | Default | Extra |
  +--------+--------------+------+-----+---------+-------+
  | id     | int(11)      | NO   | PRI | NULL    |       |
  | w_name | varchar(256) | NO   |     | NULL    |       |
  +--------+--------------+------+-----+---------+-------+
  2 rows in set
  

• For a column with the NOT NULL constraint, you must specify the value in an INSERT statement unless you have defined a default value for the column.

  obclient> INSERT INTO ware(id) values(1);
  ERROR 1364 (HY000): Field 'w_name' doesn't have a default value
  

• For example, if you define a default value 0.99 for the c_discount column in the cust table, the default discounted price is 99% of the original price.

  obclient> CREATE TABLE cust(          
              id int,          
              c_discount varchar(256) default '0.99',          
              PRIMARY KEY (id)          
             );
  Query OK, 0 rows affected
  
  obclient> DESC cust;
  +------------+--------------+------+-----+---------+-------+
  | Field      | Type         | Null | Key | Default | Extra |
  +------------+--------------+------+-----+---------+-------+
  | id         | int(11)      | NO   | PRI | NULL    |       |
  | c_discount | varchar(256) | YES  |     | 0.99    |       |
  +------------+--------------+------+-----+---------+-------+
  2 rows in set
  

Example 2

• The UNIQUE constraint on the w_name and w_city columns in the ware table requires that the warehouse names in each city must be unique.

  obclient> CREATE TABLE ware(
              w_id int,
              w_name varchar(256) unique,
              w_city varchar(256) unique,
              PRIMARY KEY (w_id)
                  ) ;
  Query OK, 0 rows affected
  
  obclient> DESC ware;
  +--------+--------------+------+-----+---------+-------+
  | Field  | Type         | Null | Key | Default | Extra |
  +--------+--------------+------+-----+---------+-------+
  | w_id   | int(11)      | NO   | PRI | NULL    |       |
  | w_name | varchar(256) | YES  | UNI | NULL    |       |
  | w_city | varchar(256) | YES  | UNI | NULL    |       |
  +--------+--------------+------+-----+---------+-------+
  3 rows in set
  

• If the w_name and w_city columns in the ware table violate the UNIQUE constraint, an error is reported.

  obclient> SELECT * FROM ware;
  +------+--------+--------+
  | w_id | w_name | w_city |
  +------+--------+--------+
  |    1 | ny     | LA     |
  +------+--------+--------+
  1 row in set
  
  obclient> INSERT INTO ware VALUES (2,'ny','LA');
  ERROR 1062 (23000): Duplicate entry 'ny' for key 'w_name'
  

Example 3

• The ware table has a primary key column w_id, and this column must contain unique values and must not contain null values.

  • If the primary key column contains null values, an error is reported.

    obclient> INSERT INTO ware(w_name,w_city) VALUES ('ny', 'LA');
    ERROR 1364 (HY000): Field 'w_id' doesn't have a default value
    

  • If the primary key column contains duplicate values, an error is reported.

    obclient> SELECT * FROM ware;
    +------+--------+--------+
    | w_id | w_name | w_city |
    +------+--------+--------+
    |    1 | ny     | LA     |
    +------+--------+--------+
    1 row in set (0.00 sec)
    
    obclient> INSERT INTO ware VALUES (1,'zs','EH');
    ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
    

    Note

    In MySQL mode, the NOT NULL constraint is enabled for the primary key column by default.

Example 4

The FOREIGN KEY constraint on the c_w_id column of the cust table references the w_id column of the ware table. This means the warehouses allocated to the customers must be warehouses in the ware table.

obclient> ALTER TABLE cust ADD CONSTRAINT ware FOREIGN KEY (c_w_id) REFERENCES ware(w_id);
Query OK, 0 rows affected

Default setting for a date, time, or datetime column

We recommend that you specify a default value for a column with the NOT NULL constraint. For a date, time, or datetime column, you can set the current date or time of the database as the default value.

Example

Call the current_timestamp function to set a default value for each datetime column in a table.

1. Create a table named t1 and define the current_timestamp function.

   obclient> create table t1(
       id bigint not null primary KEY
       , gmt_create datetime not null default current_timestamp 
       , gmt_modified datetime not null default current_timestamp 
   );
   Query OK, 0 rows affected
   

2. Insert data into the t1 table.

   obclient> insert into t1(id) values(1),(2),(3);
   Query OK, 3 rows affected (0.01 sec)
   Records: 3  Duplicates: 0  Warnings: 0
   

3. View the t1 table.

   obclient> select * from t1;
   +----+---------------------+---------------------+
   | id | gmt_create          | gmt_modified        |
   +----+---------------------+---------------------+
   |  1 | 2022-03-16 14:23:10 | 2022-03-16 14:23:10 |
   |  2 | 2022-03-16 14:23:10 | 2022-03-16 14:23:10 |
   |  3 | 2022-03-16 14:23:10 | 2022-03-16 14:23:10 |
   +----+---------------------+---------------------+
   3 rows in set