unique就是限制字段的值唯一用的 create table t2(x int unique); #将字段x这只为唯一,也就是说不能插入的值不能重复喽 create table t3( x int, y varchar(5), unique key(x) #换汤不换药,只是换了件衣服,变换一种形式告诉你,原来设置字段唯一还可以这用,呵呵并没有什么卵用 ); '''---''' cr...
mysql> create table tb_dept3(id int(11)primary key,name varchar(22),location varchar(50),constraint sth unique(name)); 1. unique和primary key的区别:一个表中可以有多个字段声明为unique,但只能有一个primary key声明;声明为primary key的列不允许有空值,但是声明为unique的字段允许空值的存在。 6.默...
pwdvarchar(20),constraintt1PRIMARYKEY(name,pwd)-- 自定义,主键约束名为t1);-- 该语句,查询出,test_cpe_pri的主键约束名为primary,可见主键自定义约束名,也不会生效SELECT*FROMinformation_schema.`TABLE_CONSTRAINTS`WHERETABLE_NAME='test_cpe_pri';INSERTINTOtest_cpe_pri(id,`name`,`pwd`)VALUES(1,'win...
MySQL中创建唯一约束同样有两种方法:创建表时指定唯一约束:create table 表名(列名1 类型unique,列名2 类型); 创建表之后指定唯一约束:alter table 表名 add constraint unique_pn unique(列名); 这里constraint 约束名 不省略,为后面的删除唯一约束做准备。 create table student1(id int constraint unique_pn uniq...
TO 为可选参数,使用与否均不影响结果。 修改表的字符集 altertable student character set utf8; 修改字段类型和列级约束 ALTER TABLE studentinfo MODIFY COLUMN borndate DATE ; 添加字段 ALTER TABLE studentinfo ADD COLUMN email VARCHAR(20) first; ...
For more information, see Section 13.1.18.3, “CREATE TABLE ... LIKE Statement”. [AS] query_expression To create one table from another, add a SELECT statement at the end of the CREATE TABLE statement: CREATE TABLE new_tbl AS SELECT * FROM orig_tbl; For more information, see Secti...
Now, I need to add a 5th column to the table and define a new unique constraint, comprising the 3 old columns and the new column. So my strategy is to drop the unique constraint, add a new column to the table, and then set the new constraints. I have no primary key defined on th...
Enable primary key and unique key supplemental logging at the database level: exec rdsadmin.rdsadmin_util.alter_supplemental_logging('ADD', 'PRIMARY KEY'); exec rdsadmin.rdsadmin_util.alter_supplemental_logging('ADD', 'UNIQUE'); Enable table-level supplemental lo...
Yes, we can remove a primary key constraint from a table. We will have to use the Alter table ..Drop the statement Syntax: ALTER TABLE <table-name> drop primary key; 23. How to add a column to an existing table? We can use ALTER TABLE and ADD COLUMN command to add a column in ...
I have a table ‘keywords’, and want to add 2 different indexes forkeywordfield in the table: 1 is for its unique (no duplicated keyword), 2 is for speeding up (in the case of input-field auto-complete). Like this: alter table `keywords` add unique index (`keyword`); ...