为此，请使用 SET yourColumnName = NULL，如下面的语法所示 -update yourTableName set yourColumnName=NULL where yourColumnName=yourValue;让我们首先创建一个表 -mysql> create table DemoTable1914    (    Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,    Code varchar(20)    )AUTO_INCREMENT=1001; Query OK, 0 rows affected (0.00 sec)插入一些记录到表中使用 insert 命令 -mysql> insert into DemoTable1914(Code) values('John101'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1914(Code) values('234David'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1914(Code) values('100_Mike'); Query OK, 1 row affected (0.00 sec)显示表中所有记录使用 select ... 阅读更多

要追加 000，请使用 ZEROFILL 的概念。让我们首先创建一个表 -mysql> create table DemoTable1913    (    Code int(4) ZEROFILL AUTO_INCREMENT NOT NULL,    PRIMARY KEY(Code)    ); Query OK, 0 rows affected (0.00 sec)插入一些记录到表中使用 insert 命令 -mysql> insert into DemoTable1913 values(1); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1913 values(2); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1913 values(3); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1913 values(4); Query OK, 1 row affected (0.00 sec)显示表中所有记录 ... 阅读更多

为此，请使用 ORDER BY 以及 LIMIT。让我们首先创建一个表，其中包含一个包含用户 ID、登录时间和名称的列 -mysql> create table DemoTable1911    (    UserId int,    UserLoggedInTime time,    UserName varchar(20)    ); Query OK, 0 rows affected (0.00 sec)插入一些记录到表中使用 insert 命令 -mysql> insert into DemoTable1911 values(100, '7:32:00', 'Chris'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1911 values(101, '5:00:00', 'David'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1911 values(102, '6:10:20', 'Mike'); Query OK, 1 row affected (0.00 ... 阅读更多

为此，您可以使用 CASE WHEN 语句。让我们首先创建一个表 -mysql> create table DemoTable1910    (    FirstName varchar(20),    Marks int    ); Query OK, 0 rows affected (0.00 sec)插入一些记录到表中使用 insert 命令 -mysql> insert into DemoTable1910 values('Chris', 45); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1910 values('David', 85); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1910 values('Chris', 55); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1910 values('Chris', 98); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1910 values('David', ... 阅读更多

要返回数据库列表，语法如下 -select schema_name as anyAliasName from information_schema.schemata;以下是返回 MySQL 中数据库列表的查询 -mysql> select schema_name as DatabaseName from information_schema.schemata;这将产生以下输出 -+---------------------------+ | DatabaseName              | +---------------------------+ | mysql                     | | information_schema        | | performance_schema        | | sys                       | | business                  | | sample                    | | hello                     | | test                      | | mybusiness                | | databasesample            | | schemasample                | | universitydatabase        | | education                 | | mydatabase                | | database1                 | | sampledatabase            | | test3                     | | javadatabase2             | | javasampledatabase        | | rdb                       | | onetomanyrelationship     | | webtracker                | | web                       | | commandline               | | hb_student_tracker        | | bothinnodbandmyisam       | | customertracker           | | tracker                   | | demo                      | | customer_tracker_database | | login                     | | onlinebookstore           | | customer-tracker          | | web_tracker               | | instant_app               | | 1233                      | +---------------------------+ 36 rows in set (0.00 sec)

为此，您只需要使用 SET 命令一次。让我们首先创建一个表 -mysql> create table DemoTable1909    (    Id int NOT NULL,    FirstName varchar(20),    LastName varchar(20)    ); Query OK, 0 rows affected (0.00 sec)插入一些记录到表中使用 insert 命令 -mysql> insert into DemoTable1909 values(101, 'John', 'Smith'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1909 values(102, 'John', 'Doe'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1909 values(103, 'Adam', 'Smith'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1909 values(104, 'David', 'Miller'); Query ... 阅读更多

让我们首先创建一个表 -mysql> create table DemoTable1908    (    Code text    ); Query OK, 0 rows affected (0.00 sec)插入一些记录到表中使用 insert 命令 -mysql> insert into DemoTable1908 values('MySQL(1)Database'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1908 values('MongoDB 2 Database'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1908 values('MySQL(3)Database'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1908 values('SQL Server(10)Database'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1908 values('MySQL 8 Database'); Query OK, 1 row affected (0.00 sec)显示所有记录从 ... 阅读更多

让我们首先创建一个表 -mysql> create table DemoTable1907    (    UserId int NOT NULL AUTO_INCREMENT,    UserName varchar(20),    UserAge int,    UserCountryName varchar(20),    PRIMARY KEY(UserId)    )ENGINE=MyISAM, AUTO_INCREMENT=100; Query OK, 0 rows affected (0.00 sec)插入一些记录到表中使用 insert 命令 -mysql> insert into DemoTable1907(UserName, UserAge, UserCountryName) values('Chris', 26, 'US'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1907(UserName, UserAge, UserCountryName) values('David', 38, 'UK'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1907(UserName, UserAge, UserCountryName) values('John', 28, 'AUS'); Query OK, 1 row affected (0.00 sec)显示所有记录从 ... 阅读更多

要更改日期的格式，请使用 DATE_FORMAT() 函数。 让我们先创建一个表 -mysql> create table DemoTable1906    (    DueTime datetime    ); Query OK, 0 rows affected (0.00 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable1906 values(now()); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1906 values(curdate()); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1906 values('2017-10-11'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1906 values('2015-01-12'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1906 values('2018-04-25'); Query OK, 1 row affected (0.00 ... 阅读更多

要加快进程，可以使用索引。 让我们先创建一个表 -mysql> create table DemoTable1905    (    FirstName varchar(20),    LastName varchar(20) ,    INDEX F_L_Name(FirstName, LastName)    ); Query OK, 0 rows affected (0.00 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable1905 values('John', 'Smith'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1905 values('John', 'Doe'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1905 values('Adam', 'Smith'); Query OK, 1 row affected (0.00 sec) mysql> insert into DemoTable1905 values('John', 'Doe'); Query OK, 1 row affected (0.00 sec) ... 阅读更多