In a company database, this table could store employee information, ensuring that each employee has a unique ID, a valid name, a positive age, and a precise salary value. Additional Notes:Defining primary keys and constraints helps in organizing data efficiently and avoiding redundancy. Scenar...
使用SHOW CREATE TABLE命令 查看information_schema.TABLE_CONSTRAINTS视图 查看information_schema.CHECK_CONSTRAINTS视图 constraint_name约束名称,最多包含 64 个字符。 约束名称的开头结尾中间都允许有空格,但需要用"`"标识名称的开头和结尾。 约束名称可以包含特殊字符"$"。
可以在创建表时规定约束(通过 CREATE TABLE 语句),或者在表创建之后也可以(通过 ALTER TABLE 语句)。 我们将主要探讨以下几种约束: NOT NULL UNIQUE PRIMARY KEY FOREIGN KEY CHECK DEFAULT SQL NOT NULL 约束 NOT NULL 约束强制列不接受 NULL 值。 SQL UNIQUE 约束 SQL UNIQUE 约束 UNIQUE 约束唯一标识数据库表...
MySQL常见的建表约束(Create Table Constraints) 一、MySQL约束类型 约束名称 描述 NOT NULL 非空约束 UNIQUE 唯一约束,取值不允许重复, PRIMARY KEY 主键约束(主关键字),自带非空、唯一、索引 FOREIGN KEY 外键约束(外关键字)
Simple CREATE TABLE syntax (common if not using options): syntaxsql Copy CREATE TABLE { database_name.schema_name.table_name | schema_name.table_name | table_name } ( { <column_definition> } [ ,... n ] ) [ ; ] Full syntax Disk-based CREATE TABLE syntax: syntaxsql Copy CREATE...
Modify a table To modify a table, you use an ALTER TABLE command. You can use an ALTER TABLE command to add, modify, or drop (remove) columns or constraints. An ALTER TABLE command has the following syntax: ALTER TABLE table_name predicate ...
当执行 INSERT 和 UPDATE 语句时,CHECK CONSTRAINTS 和规则具有相同的数据验证功能。 当列上存在规则和一个或多个 CHECK 约束时,将验证所有限制。 不能在 text、ntext 或 image 列上定义 CHECK 约束。 其他约束信息 为约束创建的索引不能用 DROP INDEX 删除;必须用 ALTER TABLE 删除约束。可以用 ALTER INDEX 重...
Before creating any table in the database, there is a need to design the tables that what kind of information will be stored in a table. What are columns needed, and what will be the datatypes of columns? What constraints will be applicable on different columns? What constraints should be...
CREATE TABLE http://download.oracle.com/docs/cd/E11882_01/server.112/e17118/statements_7002.htm#SQLRF01402 表空间(tableSpace) 段(segment) 盘区(extent) 块(block) 关系 一. Storage 参数说明 1. INITIAL Specify the size of the first extent of the object. Oracle allocates spac...
( parent_table [, ... ] ) ] [ PARTITION BY { RANGE | LIST | HASH } ( { column_name | ( expression ) } [ COLLATE collation ] [ opclass ] [, ... ] ) ] [ USING method ] [ WITH ( storage_parameter [= value ] [, ... ] ) | WITHOUT OIDS ] [ ON COMMIT { PRESERVE ...