要使一对列唯一，请使用 UNIQUE 和 ALTER TABLE 命令。以下是语法 -alter table yourTableName add unique yourUniqueName(yourColumnName1, yourColumnName2, ...N);让我们先创建一个表 -mysql> create table DemoTable (    StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY,    StudentFirstName varchar(100),    StudentLastName varchar(100),    StudentAge int,    StudentPhoneNumber varchar(20) ); Query OK, 0 rows affected (0.81 sec)以下是使 MySQL 中一对列唯一的查询 -mysql> alter table DemoTable add unique DemoTable_unique_StudentFirstName_StudentPhoneNumber(StudentFirstName, StudentPhoneNumber); Query OK, 0 rows affected (0.40 sec) Records: 0 Duplicates: 0 Warnings: 0插入一些记录到 ... 阅读更多

让我们首先看看这种情况何时会出现。创建一个表并设置列名和数据类型，但不设置大小 -mysql> create table DemoTable (    Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,    FirstName varchar,    LastName varchar ); ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ', LastName varchar )' at line 4您可以通过为 varchar 数据类型提供大小（如 varchar(100)）来更正上述错误。这将解决问题。让我们修复它 ... 阅读更多

要使用保留字“Key”，请使用反引号符号的概念。在这里，举个例子，我使用列名 key，它需要在列名周围使用反引号符号。让我们先创建一个表 -mysql> create table DemoTable (    `Key` int ); Query OK, 0 rows affected (0.67 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable values(100); Query OK, 1 row affected (0.16 sec) mysql> insert into DemoTable values(101); Query OK, 1 row affected (0.55 sec) mysql> insert into DemoTable values(110); Query OK, 1 row affected (0.28 sec) mysql> insert ... 阅读更多

让我们先创建一个表 -mysql> create table DemoTable (    Money DECIMAL(7, 2) ); Query OK, 0 rows affected (0.58 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable values(100.67); Query OK, 1 row affected (0.16 sec) mysql> insert into DemoTable values(199.33); Query OK, 1 row affected (0.12 sec) mysql> insert into DemoTable values(500); Query OK, 1 row affected (0.19 sec) mysql> insert into DemoTable values(400); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable values(800); Query OK, 1 row affected (0.14 sec)使用 select 语句显示表中的所有记录 -mysql> ... 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 DemoTable (    Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,    Value int ); Query OK, 0 rows affected (0.52 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable(Value) values(600); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable(Value) values(600); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable(Value) values(800); Query OK, 1 row affected (0.12 sec) mysql> insert into DemoTable(Value) values(800); Query OK, 1 row affected (0.09 sec)显示表中的所有记录 ... 阅读更多

为此，请使用 JSON_OBJECTAGG()。让我们先创建一个表 -mysql> create table DemoTable (    Id int,    FirstName varchar(100),    Age int ); Query OK, 0 rows affected (0.56 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable values(10, 'John', 23); Query OK, 1 row affected (0.18 sec) mysql> insert into DemoTable values(20, 'Carol', 21); Query OK, 1 row affected (0.13 sec) mysql> insert into DemoTable values(10, 'Sam', 24); Query OK, 1 row affected (0.15 sec) mysql> insert into DemoTable values(20, 'Chris', 20); Query OK, 1 row affected (0.13 sec)显示所有记录 ... 阅读更多

让我们先创建一个表 -mysql> create table DemoTable (    Logouttime time ); 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 DemoTable values('5:50:00'); Query OK, 1 row affected (0.15 sec) mysql> insert into DemoTable values('6:10:10'); Query OK, 1 row affected (0.17 sec) mysql> insert into DemoTable values('8:00:50'); Query OK, 1 row affected (0.14 sec)使用 select 语句显示表中的所有记录 -mysql> select *from DemoTable;这将产生以下输出 -+------------+ | Logouttime | +------------+ | 05:50:00 | | 06:10:10 | | 08:00:50 ... 阅读更多

为此，您可以使用聚合函数 SUM()。让我们先创建一个表 -mysql> create table DemoTable (    isMarried tinyint(1) ); Query OK, 0 rows affected (0.84 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable values(0); Query OK, 1 row affected (0.26 sec) mysql> insert into DemoTable values(1); Query OK, 1 row affected (0.11 sec) mysql> insert into DemoTable values(1); Query OK, 1 row affected (0.21 sec) mysql> insert into DemoTable values(0); Query OK, 1 row affected (0.20 sec) mysql> insert into DemoTable values(1); Query OK, 1 row affected (0.19 sec) mysql> ... 阅读更多

假设您已将日期设置为 VARCHAR 格式。现在，如果您想更新格式，则可以使用 UPDATE 命令以及 STR_TO_DATE()。语法如下 -update yourTableName set yourColumnName=str_to_date(yourColumnName, '%m/%d/%Y');让我们先创建一个表 -mysql> create table DemoTable (    DueDate varchar(100) ); Query OK, 0 rows affected (0.56 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable values('12/01/2019'); Query OK, 1 row affected (0.17 sec) mysql> insert into DemoTable values('01/31/2016'); Query OK, 1 row affected (0.16 sec) mysql> insert into DemoTable values('03/17/2018'); Query OK, 1 row affected (0.17 ... 阅读更多

让我们先创建一个表 -mysql> create table DemoTable (    isValidUser boolean ); Query OK, 0 rows affected (0.70 sec)使用 insert 命令在表中插入一些记录 -mysql> insert into DemoTable values(true); Query OK, 1 row affected (0.19 sec) mysql> insert into DemoTable values(false); Query OK, 1 row affected (0.07 sec) mysql> insert into DemoTable values(false); Query OK, 1 row affected (0.10 sec) mysql> insert into DemoTable values(true); Query OK, 1 row affected (0.11 sec) mysql> insert into DemoTable values(true); Query OK, 1 row affected (0.13 sec) mysql> insert into DemoTable values(true); Query OK, 1 row affected ... 阅读更多