- Introduction
- Concepts
- Architecture
- Key Features
- Horizontal Scalability
- MySQL Compatible Syntax
- Replicate from and to MySQL
- Distributed Transactions with Strong Consistency
- Cloud Native Architecture
- Minimize ETL with HTAP
- Fault Tolerance & Recovery with Raft
- Automatic Rebalancing
- Deployment and Orchestration with Ansible, Kubernetes, Docker
- JSON Support
- Spark Integration
- Read Historical Data Without Restoring from Backup
- Fast Import and Restore of Data
- Hybrid of Column and Row Storage
- SQL Plan Management
- Open Source
- Online Schema Changes
- How-to
- Get Started
- Deploy
- Hardware Recommendations
- From Binary Tarball
- Orchestrated Deployment
- Geographic Redundancy
- Data Migration with Ansible
- Configure
- Secure
- Transport Layer Security (TLS)
- Generate Self-signed Certificates
- Monitor
- Migrate
- Maintain
- Common Ansible Operations
- Backup and Restore
- Identify Abnormal Queries
- Scale
- Upgrade
- Troubleshoot
- Reference
- SQL
- MySQL Compatibility
- SQL Language Structure
- Data Types
- Functions and Operators
- Function and Operator Reference
- Type Conversion in Expression Evaluation
- Operators
- Control Flow Functions
- String Functions
- Numeric Functions and Operators
- Date and Time Functions
- Bit Functions and Operators
- Cast Functions and Operators
- Encryption and Compression Functions
- Information Functions
- JSON Functions
- Aggregate (GROUP BY) Functions
- Window Functions
- Miscellaneous Functions
- Precision Math
- List of Expressions for Pushdown
- SQL Statements
ADD COLUMN
ADD INDEX
ADMIN
ADMIN CANCEL DDL
ADMIN CHECKSUM TABLE
ADMIN CHECK [TABLE|INDEX]
ADMIN SHOW DDL [JOBS|QUERIES]
ALTER DATABASE
ALTER TABLE
ALTER USER
ANALYZE TABLE
BEGIN
CHANGE COLUMN
COMMIT
CREATE DATABASE
CREATE INDEX
CREATE ROLE
CREATE TABLE LIKE
CREATE TABLE
CREATE USER
CREATE VIEW
DEALLOCATE
DELETE
DESC
DESCRIBE
DO
DROP COLUMN
DROP DATABASE
DROP INDEX
DROP ROLE
DROP TABLE
DROP USER
DROP VIEW
EXECUTE
EXPLAIN ANALYZE
EXPLAIN
FLUSH PRIVILEGES
FLUSH STATUS
FLUSH TABLES
GRANT <privileges>
GRANT <role>
INSERT
KILL [TIDB]
LOAD DATA
LOAD STATS
MODIFY COLUMN
PREPARE
RECOVER TABLE
RENAME INDEX
RENAME TABLE
REPLACE
REVOKE <privileges>
REVOKE <role>
ROLLBACK
SELECT
SET DEFAULT ROLE
SET [NAMES|CHARACTER SET]
SET PASSWORD
SET ROLE
SET TRANSACTION
SET [GLOBAL|SESSION] <variable>
SHOW ANALYZE STATUS
SHOW CHARACTER SET
SHOW COLLATION
SHOW [FULL] COLUMNS FROM
SHOW CREATE TABLE
SHOW CREATE USER
SHOW DATABASES
SHOW ENGINES
SHOW ERRORS
SHOW [FULL] FIELDS FROM
SHOW GRANTS
SHOW INDEXES [FROM|IN]
SHOW INDEX [FROM|IN]
SHOW KEYS [FROM|IN]
SHOW PRIVILEGES
SHOW [FULL] PROCESSSLIST
SHOW SCHEMAS
SHOW STATUS
SHOW [FULL] TABLES
SHOW TABLE REGIONS
SHOW TABLE STATUS
SHOW [GLOBAL|SESSION] VARIABLES
SHOW WARNINGS
SPLIT REGION
START TRANSACTION
TRACE
TRUNCATE
UPDATE
USE
- Constraints
- Generated Columns
- Partitioning
- Character Set
- SQL Mode
- Views
- Configuration
- Security
- Transactions
- System Databases
- Errors Codes
- Supported Client Drivers
- Garbage Collection (GC)
- Performance
- Overview
- Understanding the Query Execution Plan
- The Blocklist of Optimization Rules and Expression Pushdown
- Introduction to Statistics
- TopN and Limit Push Down
- Optimizer Hints
- Check the TiDB Cluster Status Using SQL Statements
- Execution Plan Binding
- Statement Summary Table
- Tune TiKV
- Operating System Tuning
- Column Pruning
- Key Monitoring Metrics
- Alert Rules
- Best Practices
- TiSpark
- TiKV
- TiDB Binlog
- Tools
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- FAQs
- Support
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- Releases
- All Releases
- v3.0
- v2.1
- v2.0
- v1.0
- Glossary
You are viewing the documentation of an older version of the TiDB database (TiDB v3.0).
Constraints
TiDB supports almost the same constraints as MySQL.
NOT NULL
NOT NULL constraints supported by TiDB are the same as those supported by MySQL.
For example:
CREATE TABLE users (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
age INT NOT NULL,
last_login TIMESTAMP
);
INSERT INTO users (id,age,last_login) VALUES (NULL,123,NOW());
Query OK, 1 row affected (0.02 sec)
INSERT INTO users (id,age,last_login) VALUES (NULL,NULL,NOW());
ERROR 1048 (23000): Column 'age' cannot be null
INSERT INTO users (id,age,last_login) VALUES (NULL,123,NULL);
Query OK, 1 row affected (0.03 sec)
- The first
INSERT
statement succeeds because it is possible to assignNULL
to theAUTO_INCREMENT
column. TiDB generates sequence numbers automatically. - The second
INSERT
statement fails because theage
column is defined asNOT NULL
. - The third
INSERT
statement succeeds because thelast_login
column is not explicitly defined asNOT NULL
. NULL values are allowed by default.
CHECK
TiDB parses but ignores CHECK
constraints. This is MySQL 5.7 compatible behavior.
For example:
DROP TABLE IF EXISTS users;
CREATE TABLE users (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
username VARCHAR(60) NOT NULL,
UNIQUE KEY (username),
CONSTRAINT min_username_length CHECK (CHARACTER_LENGTH(username) >=4)
);
INSERT INTO users (username) VALUES ('a');
SELECT * FROM users;
UNIQUE KEY
Depending on the transaction mode and the value of tidb_constraint_check_in_place
, TiDB might check UNIQUE
constraints lazily. This helps improve performance by batching network access.
For example:
DROP TABLE IF EXISTS users;
CREATE TABLE users (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
username VARCHAR(60) NOT NULL,
UNIQUE KEY (username)
);
INSERT INTO users (username) VALUES ('dave'), ('sarah'), ('bill');
With the default of pessimistic locking:
BEGIN;
INSERT INTO users (username) VALUES ('jane'), ('chris'), ('bill');
ERROR 1062 (23000): Duplicate entry 'bill' for key 'username'
With optimistic locking and tidb_constraint_check_in_place=0
:
BEGIN OPTIMISTIC;
INSERT INTO users (username) VALUES ('jane'), ('chris'), ('bill');
Query OK, 0 rows affected (0.00 sec)
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
INSERT INTO users (username) VALUES ('steve'),('elizabeth');
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
COMMIT;
ERROR 1062 (23000): Duplicate entry 'bill' for key 'username'
In the optimistic example, the unique check was delayed until the transaction is committed. This resulted in a duplicate key error, because the value bill
was already present.
You can disable this behavior by setting tidb_constraint_check_in_place
to 1
. This variable setting does not take effect on pessimistic transactions, because in the pessimistic transaction mode the constraints are always checked when the statement is executed. When tidb_constraint_check_in_place=1
, the unique constraint is checked when the statement is executed.
For example:
DROP TABLE IF EXISTS users;
CREATE TABLE users (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
username VARCHAR(60) NOT NULL,
UNIQUE KEY (username)
);
INSERT INTO users (username) VALUES ('dave'), ('sarah'), ('bill');
SET tidb_constraint_check_in_place = 1;
Query OK, 0 rows affected (0.00 sec)
BEGIN OPTIMISTIC;
Query OK, 0 rows affected (0.00 sec)
INSERT INTO users (username) VALUES ('jane'), ('chris'), ('bill');
ERROR 1062 (23000): Duplicate entry 'bill' for key 'username'
..
The first INSERT
statement caused a duplicate key error. This causes additional network communication overhead and might reduce the throughput of INSERT
operations.
PRIMARY KEY
Like MySQL, primary key constraints contain unique constraints, that is, creating a primary key constraint is equivalent to having a unique constraint. In addition, other primary key constraints of TiDB are also similar to those of MySQL.
For example:
CREATE TABLE t1 (a INT NOT NULL PRIMARY KEY);
Query OK, 0 rows affected (0.12 sec)
CREATE TABLE t2 (a INT NULL PRIMARY KEY);
ERROR 1171 (42000): All parts of a PRIMARY KEY must be NOT NULL; if you need NULL in a key, use UNIQUE instead
CREATE TABLE t3 (a INT NOT NULL PRIMARY KEY, b INT NOT NULL PRIMARY KEY);
ERROR 1068 (42000): Multiple primary key defined
CREATE TABLE t4 (a INT NOT NULL, b INT NOT NULL, PRIMARY KEY (a,b));
Query OK, 0 rows affected (0.10 sec)
- Table
t2
failed to be created, because columna
is defined as the primary key and does not allow NULL values. - Table
t3
failed to be created, because a table can only have one primary key. - Table
t4
was created successfully, because even though there can be only one primary key, TiDB supports defining multiple columns as the composite primary key.
In addition to the rules above, by default, TiDB has an additional restriction that once a table is successfully created, its primary key cannot be changed. If you need to add or remove the primary key, set alter-primary-key
to true
in the TiDB configuration file, and restart the TiDB instance to make it effective.
When the feature of adding or deleting the primary key is enabled, TiDB allows adding the primary key to or deleting the primary key from the table. However, if a table with an integer type primary key has been created before the feature is enabled, you cannot delete its primary key constraint even when you enable the adding or deleting primary key feature.
FOREIGN KEY
TiDB has limited support for foreign key constraints.
TiDB supports creating FOREIGN KEY
constraints in DDL commands.
For example:
CREATE TABLE users (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
doc JSON
);
CREATE TABLE orders (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
user_id INT NOT NULL,
doc JSON,
FOREIGN KEY fk_user_id (user_id) REFERENCES users(id)
);
SELECT table_name, column_name, constraint_name, referenced_table_name, referenced_column_name
FROM information_schema.key_column_usage WHERE table_name IN ('users', 'orders');
+------------+-------------+-----------------+-----------------------+------------------------+
| table_name | column_name | constraint_name | referenced_table_name | referenced_column_name |
+------------+-------------+-----------------+-----------------------+------------------------+
| users | id | PRIMARY | NULL | NULL |
| orders | id | PRIMARY | NULL | NULL |
| orders | user_id | fk_user_id | users | id |
+------------+-------------+-----------------+-----------------------+------------------------+
3 rows in set (0.00 sec)
TiDB also supports the DROP FOREIGN KEY
and ADD FOREIGN KEY
syntax via the ALTER TABLE
command.
ALTER TABLE orders DROP FOREIGN KEY fk_user_id;
ALTER TABLE orders ADD FOREIGN KEY fk_user_id (user_id) REFERENCES users(id);
Notes
TiDB supports foreign keys to avoid errors caused by this syntax when you migrate data from other databases to TiDB.
However, TiDB does not perform constraint checking on foreign keys in DML statements. For example, even if there is no record with id=123 in the users table, the following transactions can be submitted successfully.
START TRANSACTION; INSERT INTO orders (user_id, doc) VALUES (123, NULL); COMMIT;