要设置 NULL 值，请将类型设置为 NULL，如下面的语法所示 -yourColumnName dataType NULL;让我们先创建一个表 -mysql> create table DemoTable759 (    Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,    FirstName varchar(100) NULL ); Query OK, 0 rows affected (0.72 sec)插入一些记录到表中使用 insert 命令 -mysql> insert into DemoTable759(FirstName) values('John'); Query OK, 1 row affected (0.17 sec) mysql> insert into DemoTable759(FirstName) values(NULL); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable759(FirstName) values('Carol'); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable759(FirstName) values(NULL); Query OK, 1 row ... 阅读更多

要设置默认值，请使用 DEFAULT 关键字。让我们先创建一个表 -mysql> create table DemoTable758 (    Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,    FirstName varchar(100) ); Query OK, 0 rows affected (0.66 sec)以下是为列设置默认值的查询 -mysql> alter table DemoTable758 add column Colors ENUM('RED', 'GREEN', 'BLUE', 'ORANGE', 'YELLOW') DEFAULT 'YELLOW'; Query OK, 0 rows affected (0.44 sec) Records: 0 Duplicates: 0 Warnings: 0让我们再次检查一下表的描述 -mysql> desc DemoTable758;这将产生以下输出 -+-----------+----------------------------------------------+------+-----+---------+----------------+ | Field     | Type     ... 阅读更多

假设您在表中有一个 VARCHAR 列，其值为字符串，数字位于右侧。例如 -John1023 Carol9871 David9098现在，假设您希望根据整个列中这些右侧的数字进行排序。为此，请使用 ORDER BY RIGHT。让我们先创建一个表 -mysql> create table DemoTable757 ( Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, ClientId varchar(100) ); Query OK, 0 rows affected (0.53 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable757(ClientId) values('John1023'); Query OK, 1 row ... 阅读更多

让我们先创建一个表 -mysql> create table DemoTable756 (    Value1 int,    Value2 int,    Value3 int ); Query OK, 0 rows affected (0.62 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable756 values(10, 20, 14); Query OK, 1 row affected (0.22 sec) mysql> insert into DemoTable756 values(20, 34, 17); Query OK, 1 row affected (0.12 sec) mysql> insert into DemoTable756 values(200, 134, 789); Query OK, 1 row affected (0.19 sec) mysql> insert into DemoTable756 values(139, 98, 99); Query OK, 1 row affected (0.17 sec)使用 select 语句显示表中的所有记录 ... 阅读更多

为此，请在 HAVING 子句中使用别名。让我们先创建一个表 -mysql> create table DemoTable755 (    Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,    Score1 int,    Score2 int ); Query OK, 0 rows affected (0.62 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable755(Score1, Score2) values(30, 23); Query OK, 1 row affected (0.18 sec) mysql> insert into DemoTable755(Score1, Score2) values(50, 60); Query OK, 1 row affected (0.13 sec) mysql> insert into DemoTable755(Score1, Score2) values(89, 90); Query OK, 1 row affected (0.29 sec) mysql> insert into DemoTable755(Score1, Score2) values(99, 99); Query OK, 1 row ... 阅读更多

要仅返回不同的值，请使用 GROUP BY 子句。让我们先创建一个表 -mysql> create table DemoTable754 (ProductPrice int); Query OK, 0 rows affected (0.48 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable754 values(200); Query OK, 1 row affected (0.20 sec) mysql> insert into DemoTable754 values(500); Query OK, 1 row affected (0.10 sec) mysql> insert into DemoTable754 values(200); Query OK, 1 row affected (0.11 sec) mysql> insert into DemoTable754 values(500); Query OK, 1 row affected (0.10 sec) mysql> insert into DemoTable754 values(800); Query OK, 1 row affected (0.24 sec) mysql> insert into DemoTable754 ... 阅读更多

要仅提取月份部分，请使用 DATE_FORMAT()。让我们先创建一个表 -mysql> create table DemoTable753 (DueDate datetime); Query OK, 0 rows affected (0.51 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable753 values('2019-06-21'); Query OK, 1 row affected (0.15 sec) mysql> insert into DemoTable753 values('2019-01-31'); Query OK, 1 row affected (0.18 sec) mysql> insert into DemoTable753 values('2018-12-01'); Query OK, 1 row affected (0.13 sec) mysql> insert into DemoTable753 values('2016-11-11'); Query OK, 1 row affected (0.23 sec)使用 select 语句显示表中的所有记录 -mysql> select *from DemoTable753;这将产生以下输出 -+---------------------+ ... 阅读更多

要实现自定义排序的 IN()，请使用 ORDER BY CASE。让我们先创建一个表 -mysql> create table DemoTable752 (    Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,    Name varchar(100) ); Query OK, 0 rows affected (0.63 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable752(Name) values('John'); Query OK, 1 row affected (0.15 sec) mysql> insert into DemoTable752(Name) values('Carol'); Query OK, 1 row affected (0.20 sec) mysql> insert into DemoTable752(Name) values('Bob'); Query OK, 1 row affected (0.12 sec) mysql> insert into DemoTable752(Name) values('Mike'); Query OK, 1 row affected (0.23 sec) mysql> insert into DemoTable752(Name) values('Sam'); ... 阅读更多

为此，您可以使用 GROUP BY HAVING 子句。让我们先创建一个表 -mysql> create table DemoTable751 (    StudentName varchar(100),    SubjectName varchar(100) ); Query OK, 0 rows affected (0.66 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable751 values('John', 'MySQL'); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable751 values('John', 'MongoDB'); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable751 values('Sam', 'MySQL'); Query OK, 1 row affected (0.17 sec) mysql> insert into DemoTable751 values('Carol', 'Java'); Query OK, 1 row affected (0.21 sec) mysql> insert into DemoTable751 values('David', 'MySQL'); ... 阅读更多

假设表中某些列的值包含特定关键字，并且您只需要这些记录。为此，请使用 LIKE 运算符。让我们先看一个例子并创建一个表 -mysql> create table DemoTable750 (Title varchar(200)); Query OK, 0 rows affected (0.69 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable750 values('Java and MongoDB, MySQL'); Query OK, 1 row affected (0.13 sec) mysql> insert into DemoTable750 values('MySQL, SQL Server'); Query OK, 1 row affected (0.21 sec) mysql> insert into DemoTable750 values('PL/SQL, Python, MongoDB'); Query OK, 1 row affected (0.14 sec) mysql> ... 阅读更多