为此，您可以使用 CONCAT_WS()。让我们创建一个表 - mysql> create table demo38 -> ( -> user_id int, -> user_first_name varchar(20), -> user_last_name varchar(20), -> user_date_of_birth date -> ); Query OK, 0 rows affected (1.70 sec) 使用 insert 命令将一些记录插入表中 - mysql> insert into demo38 values(10, 'John', 'Smith', '1990−10−01'); Query OK, 1 row affected (0.14 sec) mysql> insert into demo38 values(11, 'David', 'Miller', '1994−01−21'); Query OK, 1 row affected (0.13 sec) mysql> insert into demo38 values(11, 'John', 'Doe', '1992−02−01'); Query OK, 1 row affected (0.13 sec) mysql> insert into ... 阅读更多

为此，您可以在 Java 中使用 PrepareStatement。以下是语法 - String anyVariableName="select yourColumnName from yourTableName where name = ?"; PreparedStatement ps = (PreparedStatement) con.prepareStatement(yourVariableName); ps.setString(yourColumnIndex, yourValue); 让我们创建一个表 - mysql> create table demo37 -> ( -> id int not null auto_increment primary key, -> name varchar(200) -> ); Query OK, 0 rows affected (2.46 sec) 使用 insert 命令将一些记录插入表中 - mysql> insert into demo37(name) values('John'); Query OK, 1 row affected (0.09 sec) mysql> insert into demo37(name) values('Bob'); Query OK, 1 row affected (0.08 sec) mysql> insert into demo37(name) values('John'); Query OK, ... 阅读更多

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 -> ); Query OK, 0 rows affected (1.68 sec) 使用 insert 命令将一些记录插入表中 - mysql> insert into demo36(value1, value2, value3) values(1, 0, 0); Query OK, 1 row affected (0.14 sec) mysql> insert into demo36(value1, value2, value3) values(1, 0, 1); Query OK, 1 row affected (0.11 sec) 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 -> ); Query OK, 0 rows affected (3.88 sec) 使用 insert 命令将一些记录插入表中 - mysql> insert into demo35(joining_date, relieving_date) values('2020−01−10', '2020−07−11'); Query OK, 1 row affected (0.15 sec) mysql> insert into demo35(joining_date, relieving_date) values('2020−05−07', '2020−12−08'); Query OK, 1 row affected (0.17 sec) mysql> insert into demo35(joining_date, relieving_date) values('2020−04−11', '2020−09−18'); Query OK, ... 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) -> ); Query OK, 0 rows affected (1.51 sec) 使用 insert 命令将一些记录插入表中 - mysql> insert into demo34 values('31−Jan−19'); Query OK, 1 row affected (0.13 sec) mysql> insert into demo34 values('03−Mar−21'); Query OK, 1 row affected (0.12 sec) mysql> insert into demo34 values('13−Jun−20'); Query OK, 1 row affected (0.11 sec) 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' -> ); Query OK, 0 rows affected (2.32 sec) 使用 insert 命令将一些记录插入表中 - mysql> insert into demo33(name) values('John'); Query OK, 1 row affected (0.15 sec) mysql> insert into demo33(name, end_date, category) values('David', '2020−12−21', 'Medium'); Query OK, 1 row affected (0.09 sec) 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 -> ); Query OK, 0 rows affected (1.42 sec) 使用 insert 命令将一些记录插入表中 - mysql> insert into demo32 values(30, 60); Query OK, 1 row affected (0.16 sec) mysql> insert into demo32 values(20, 40); Query OK, 1 row affected (0.15 sec) mysql> insert into demo32 values(35, 35); Query OK, 1 row affected (0.08 sec) 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 -> ); Query OK, 0 rows affected (2.27 sec) 使用 insert 命令将一些记录插入表中 - mysql> insert into demo31 values(34, 55, 67); Query OK, 1 row affected (0.27 sec) mysql> insert into demo31 values(50, 60, 70); Query OK, 1 row affected (0.16 sec) 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删除它 - mysql> set @currentDatabase = database(); Query OK, 0 rows affected (0.00 sec) mysql> select @currentDatabase; +------------------+ | @currentDatabase | +------------------+ | employeeonboard | +------------------+ 1 row in set (0.00 sec) mysql> set @sqlQuery = concat('drop database ', @currentDatabase); Query OK, 0 rows affected (0.00 sec) mysql> prepare ... 阅读更多

为此，请在 MySQL 中使用 IF() 函数。语法如下：