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;让我们执行上面的查询以获取当前数据库并删除它 - 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 > ); Query OK, 0 rows affected (1.67 sec)使用insert命令向表中插入一些记录：mysql> insert into demo29 values(10, 500); Query OK, 1 row affected (0.15 sec) mysql> insert into demo29 values(14, 400); Query OK, 1 row affected (0.14 sec) mysql> insert into demo29 values(9, 500); Query OK, 1 row affected (0.12 sec) 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 > ); Query OK, 0 rows affected (3.14 sec)使用insert命令向表中插入一些记录：mysql> insert into demo27(value) values(50); Query OK, 1 row affected (0.12 sec) mysql> insert into demo27(value) values(500); Query OK, 1 row affected (0.20 sec) mysql> insert into demo27(value) values(100); Query OK, 1 row affected (0.17 sec) ... 阅读更多