insert into sc (name,class,score) values ('吕布','一年二班',67); insert into sc (name,class,score) values ('赵云','一年二班',90); insert into sc (name,class,score) values ('典韦','一年二班',89); insert into sc (name,class,score) values ('关羽','一年二班',70); 1. 2. ...
SELECT X,X as Y FROM (SELECT UUID() AS X) t; I get the same value for both columns (X and Y) But when I execute SELECT X,X AS Y FROM (SELECT UUID() AS X FROM (SELECT 1 AS ZZ) z) t; I still get one row but X and Y columns have different values. ...
至于Unique Index,则只是属于Index中的一种而已,建立了Unique Index表示此列数据不可重复,猜想MySQL对Unique Index类型的索引可以做进一步特殊优化吧。 于是乎,在设计表的时候,Key只是要处于模型层面的,而当需要进行查询优化,则对相关列建立索引即可。 另外,在MySQL中,对于一个Primary Key的列,MySQL已经自动对其建立了...
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( id INT AUTO_INCREMENT PRIMARY KEY, username VARCHAR(50) NOT NULL UNIQUE, email VARCHAR(100) NOT NULL UNIQUE ); -- 插入数据 INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com'); -- 尝试插入重复的email INSERT INTO users (username, email) VALUES ('jane_doe',...
mysql> CREATE TABLE t_no_pk (c1 INT, c2 INT) -> PARTITION BY RANGE(c1) ( -> PARTITION p0 VALUES LESS THAN (10), -> PARTITION p1 VALUES LESS THAN (20), -> PARTITION p2 VALUES LESS THAN (30), -> PARTITION p3 VALUES LESS THAN (40) -> ); Query OK, 0 rows affected (0...
MySQL数据库中的unique关键字用于创建唯一约束(Unique Constraint),它确保表中某个列或一组列的值是唯一的。 概念: 唯一约束是一种数据完整性约束,它确保在列或列组中的值是唯一的,即不会重复出现。 分类: 唯一约束可以应用于单个列,也可以应用于多个列组合。
mysql> CREATE TABLE t_no_pk (c1 INT, c2 INT) -> PARTITION BY RANGE(c1) ( -> PARTITION p0 VALUES LESS THAN (10), -> PARTITION p1 VALUES LESS THAN (20), -> PARTITION p2 VALUES LESS THAN (30), -> PARTITION p3 VALUES LESS THAN (40) -> ); Query OK, 0 rows affected (0.12 ...
mysql的key和index多少有点令人迷惑,单独的key和其它关键词结合的key(primary key)实际表示的意义是不同,这实际上考察对数据库体系结构的了解的。 1 : key 是数据库的物理结构,它包含两层意义和作用, 一是约束(偏重于约束和规范数据库的结构完整性), ...
The problem is that the "DISTINCT" query takes AGES to complete. I don't actually know how long, because I stopped the query after 25 minutes. Is there a faster way for me to get unique values from a column? Sorry, you can't reply to this topic. It has been closed....