we’ll have 2 records with different ids with the same country_name. The DBMS would treat these as 2 different countries. Having the alternate key/UNIQUE defined prevents this from happening. Let’s take a look at an example. We already have the country with the...
In order to be a primary key, several conditions must hold true: The columns defining the primary key are unique. Each column may contain duplicate values; however, the combination of column values is unique. No value within the primary key columns is NULL. I would also extend this to incl...
create table a( id int primary key auto_increment, name varchar(20) not null default '' ); //这里id就是表的主键 1. 2. 3. 4. 5. 如果当创建表时没有指定主键索引,也可以在创建表之后添加: alter table table_name add primary key (column name); 1.2普通索引 普通索引一般是在建表后再添加...
handler_read_key:这个值越高越好,越高表示使用索引查询到的次数。 handler_read_rnd_next:这个值越高,说明查询低效。
Can I have a primary key as a non-unique column Can I pass parameter to an ALTER DATABASE command Can I prevent deadlock during concurrent delete Can I print to file using T- SQL Can I sort an SQL table? Can I sort row without order by clause Can I UPDATE, then INSERT if no r...
but denying the technical ability to do it at all just takes away the choice from the people who do have the competence to make a wise decision. It seems that we both agree that some keys do need to be updated but Microsoft limits our options for key generation in that case - Microsoft...
I previously blogged about the new 12c capability of having Multiple Indexes On The Same Column List and demonstrated how it was now possible to “quickly” swap the index on say a Primary Key constraint by pre-building a new index (say a Unique index to replace an existing Non-Unique ...
(mykey)); ERROR 1171: All parts of a PRIMARY KEY must be NOT NULL; If you need NULL in a key, use UNIQUE instead mysql>Suggested fix:Workaround: explicitly add NOT NULL to port to MySQL. Solution: remove error 1171 (perhaps make it a warning so that the SQL can still be used ...
Some people argue that having more than one column in a primary key “complicates things” or “makes things less efficient” rather than always using identity columns. This is simply not the case. We’ve already established that youmustadd additional unique constraints to your data to have in...
Syntax error in SQL statement "UPDATE PUBLIC.KEYVALUE SET TYPE = 'val1' WHERE [*]KEY='val2'"; expected "INTERSECTS, NOT, EXISTS, UNIQUE, INTERSECTS"; SQL statement: UPDATE PUBLIC.KEYVALUE SET TYPE = 'val1' WHERE KEY='val2' [42001-202] [Failed SQL: (42001) UPDATE PUBLIC.KEYVALUE...