INFORMATION_SCHEMA.TABLE_CONSTRAINTS是一个系统表,它存储了关于数据库中所有表的约束信息。这个表是SQL标准的一部分,大多数关系型数据库管理系统(如MySQL、PostgreSQL、SQL Server等)都提供了类似的表来存储元数据信息。 表结构 以下是INFORMATION_SCHEMA.TABLE_CONSTRAINTS表的一些主要列及其含义: CONSTRAINT_CATALOG: 约...
MySQL约束是一系列规则,用来约束(允许/限制)存储在列中的数据。施加约束的目的是为了保证数据库的一致性。 MySQL约束是在创建数据表的时候声明的。 MySQL可以实现的不同级别的约束 列级(column level)约束:作用于某一列。 表级(table level)约束:作用于整个表。 MySQL约束类型 MySQL约束的语法 CREATE TABLE [table...
ENFORCED ForCHECKconstraints, the value isYESorNOto indicate whether the constraint is enforced. For other constraints, the value is alwaysYES. This column was added in MySQL 8.0.16.
TABLE_C_ID NUMERIC NOT NULL, FIELD_A VARCHAR(200), FIELD_B VARCHAR(200), PRIMARY KEY (TABLE_C_TABLE_D_ID, TABLE_C_ID)); ALTER TABLE TABLE_D ADD ( FOREIGN KEY (TABLE_C_ID) REFERENCES TABLE_C(TABLE_C_ID)); When I query table_constraints on my TABLE_D it shows the primary ke...
4. Provide the column name, data type, and optional constraints. 5. Double-click the section under the table name to add new columns. Add as many columns as required and fill out the data for each. 6. After adding all the required data, clickApplyto generate a query that creates the ...
This is a CHAR (not ENUM) column. The CHECK value is not available until MySQL supports CHECK. The UNIQUE and PRIMARY KEY information is about the same as what you get from the Key_name column in the output from SHOW INDEX when the Non_unique column is 0. ...
SQL supports foreign keys, which let you cross-reference related data across tables, and foreign key constraints, which help keep this spread-out data consistent. The essential syntax for a foreign key constraint definition in a CREATE TABLE or ALTER TABLE statement looks like this: ...
column_constraints:列的约束条件(如NOT NULL、DEFAULT)。 示例: 将salary列的数据类型从INT更改为DECIMAL: ALTERTABLEemployees MODIFYCOLUMNsalaryDECIMAL(10,2); 4.2 修改列的默认值 要更改列的默认值,可以使用以下语法: ALTERTABLEtable_nameALTERCOLUMNcolumn_nameSETDEFAULTdefault_expression; ...
Unique indexes in MySQL ensure all values in the indexed column or columns are distinct, preventing duplicate entries. They enforce data integrity by maintaining uniqueness, essential for certain database operations and constraints. Unique indexes allow null values, but each non-null value must remain...
will change the existing TABLE_C into an INNODB table. Subject Written By Posted How to get table constraints David Baker June 26, 2007 08:14AM Re: How to get table constraints Roland Bouman August 01, 2007 06:35PM Sorry, you can't reply to this topic. It has been closed....