以下是语法：select sum(yourColumnName1+yourColumnName2+yourColumnName3...N) as `anyAliasName1`, sum(yourColumnName1 and yourColumnName2 and yourColumnName3….N) as anyAliasName from yourTableName;让我们创建一个表：mysql> create table demo36 −> ( −> id int not null auto_increment primary key, −> value1 int, −> value2 int, −> value3 int −> ); 插入一些记录到表中：mysql> insert into demo36(value1, value2, value3) values(1, 0, 0); mysql> insert into demo36(value1, value2, value3) values(1, 0, 1); mysql> insert ... 阅读更多

以下是语法：select *from yourTableName where yourColumnName1 < yourValue1 AND (yourColumnName2 > yourValue2 OR yourColumnName2 is null);让我们创建一个表：mysql> create table demo35 −> ( −> id int NOT NULL AUTO_INCREMENT PRIMARY KEY, −> joining_date date, −> relieving_date date −> ); 插入一些记录到表中：mysql> insert into demo35(joining_date, relieving_date) values('2020−01−10', '2020−07−11'); mysql> insert into demo35(joining_date, relieving_date) values('2020−05−07', '2020−12−08'); mysql> insert into demo35(joining_date, relieving_date) values('2020−04−11', '2020−09−18'); mysql> ... 阅读更多

为此，请使用 UNIX_TIMESTAMP()。以下是语法：select UNIX_TIMESTAMP(STR_TO_DATE(yourColumnName, "%d-%b-%y")) as anyAliasName from yourTableName;让我们创建一个表：mysql> create table demo34 −> ( −> datevalue varchar(40) −> ); 插入一些记录到表中：mysql> insert into demo34 values('31−Jan−19'); mysql> insert into demo34 values('03−Mar−21'); mysql> insert into demo34 values('13−Jun−20'); 使用 select 语句显示表中的记录：mysql> select *from demo34;这将产生... 阅读更多

为此，请在列数据类型之后使用 DEFAULT 关键字。让我们创建一个表：mysql> create table demo33 −> ( −> id int not null auto_increment primary key, −> name varchar(20) not null, −> start_date date default(current_date), −> end_date date default NULL, −> category enum('Good', 'Medium', 'Low') default 'Low' −> ); 插入一些记录到表中：mysql> insert into demo33(name) values('John'); mysql> insert into demo33(name, end_date, category) values('David', '2020−12−21', 'Medium'); mysql> ... 阅读更多

以下是语法：alter table yourTableName add column yourColumnName yourDataType generated always as ((yourColumName1+yourColumName2+....N) / N) virtual;让我们创建一个表：mysql> create table demo32 −> ( −> value1 int, −> value2 int −> ); 插入一些记录到表中：mysql> insert into demo32 values(30, 60); mysql> insert into demo32 values(20, 40); mysql> insert into demo32 values(35, 35); 使用... 阅读更多

为此，您可以使用 AVG()。以下是语法：select avg(yourColumnName1) as anyAliasName1, avg(yourColumnName2) as anyAliasName2, avg(yourColumnName3) as anyAliasName3, . . N from yourTableName;让我们创建一个表：mysql> create table demo31 −> ( −> value1 int, −> value2 int, −> value3 int −> ); 插入一些记录到表中：mysql> insert into demo31 values(34, 55, 67); mysql> insert into demo31 values(50, 60, 70); mysql> insert into demo31 values(100, 200, ... 阅读更多

要获取当前数据库，您可以使用 SELECT DATABASE()：select database();以下是语法：set @anyVariableName = database(); select @anyVariableName; set @anyVariableName2 = concat('drop database ', @yourVariableName); prepare anyVariableName3 from @yourVariableName2; execute yourVariableName3;让我们执行以上查询以获取当前数据库并将其删除：mysql> set @currentDatabase = database(); mysql> select @currentDatabase; mysql> set @sqlQuery = concat('drop database ', @currentDatabase); mysql> prepare ... 阅读更多

为此，请在 MySQL 中使用 IF()。语法如下：select IF(yourColumnName1=0, yourColumnName2, yourColumnName1) as anyAliasName from yourTableName;让我们创建一个表：mysql> create table demo30 −> ( −> id int not null auto_increment primary key, −> value int, −> original_value int −> ) −> ; 插入一些记录到表中：mysql> insert into demo30(value, original_value) values(50, 10); mysql> insert into demo30(value, original_value) values(1000, 0); mysql> insert into demo30(value, original_value) ... 阅读更多

以下是按多个列排序的语法：select *from yourTableName order by yourColumnName1 DESC, yourColumnName2, yourColumnName3;让我们创建一个表：mysql> create table demo29 −> ( −> value1 int, −> value2 int −> ); 插入一些记录到表中：mysql> insert into demo29 values(10, 500); mysql> insert into demo29 values(14, 400); mysql> insert into demo29 values(9, 500); mysql> insert into demo29 ... 阅读更多

为此，您需要使用 WHERE 子句。以下是语法：select *from yourTableName where yourColumnName > yourValue;让我们创建一个表：mysql> create table demo27 −> ( −> id int not null auto_increment primary key, −> value int −> ); 插入一些记录到表中：mysql> insert into demo27(value) values(50); mysql> insert into demo27(value) values(500); mysql> insert into demo27(value) values(100); ... 阅读更多