使用 CREATE TABLE IF NOT EXISTS，如下面的语法所示：create table if not exists yourTableName (    yourColumnName1 dataType,    yourColumnName2 dataType,    yourColumnName3 dataType,    .    .    N ) as select yourValue1 as yourColumnName1 , yourValue2 as yourColumnName2 , yourValue3 as yourColumnName3, .............................N;让我们首先创建一个表，如果表不存在则插入值：mysql> create table if not exists DemoTable (    id int,    FirstName varchar(20),    LastName varchar(20) ) as select 100 as id, 'John' as FirstName , 'Smith' as LastName; Query OK, 1 row ... 阅读更多

让我们首先创建一个表：mysql> create table DemoTable (    CustomerId int NOT NULL AUTO_INCREMENT PRIMARY KEY,    ProductAmount int,    CustomerCountryName varchar(10) ); Query OK, 0 rows affected (0.86 sec)使用insert命令在表中插入一些记录：mysql> insert into DemoTable(ProductAmount, CustomerCountryName) values(190, 'US'); Query OK, 1 row affected (0.23 sec) mysql> insert into DemoTable(ProductAmount, CustomerCountryName) values(200, 'UK'); Query OK, 1 row affected (0.12 sec) mysql> insert into DemoTable(ProductAmount, CustomerCountryName) values(1500, 'AUS'); Query OK, 1 row affected (0.17 sec) mysql> insert into DemoTable(ProductAmount, CustomerCountryName) values(2010, 'US'); Query OK, 1 row affected (0.13 sec) mysql> insert into ... 阅读更多

要显示单引号文本，请使用双引号，例如，如果要编写 You’ve，则在插入时编写 You've，即使用双引号。让我们首先创建一个表：mysql> create table DemoTable (    Note text ); Query OK, 0 rows affected (0.57 sec)使用insert命令在表中插入一些记录：mysql> insert into DemoTable values('You''ve seen the Taj? When?'); Query OK, 1 row affected (0.09 sec) mysql> insert into DemoTable values('You''ve visited the US?'); Query OK, 1 row affected (0.16 sec)使用select语句显示表中的所有记录：mysql> select *from DemoTable;这将产生... 阅读更多

要仅将空单元格更新为 NULL，请在 MySQL 中使用 NULLIF()。让我们首先创建一个表：mysql> create table DemoTable (    Name varchar(50) ); Query OK, 0 rows affected (1.73 sec)使用insert命令在表中插入一些记录：mysql> insert into DemoTable values('Mike'); Query OK, 1 row affected (0.07 sec) mysql> insert into DemoTable values(''); Query OK, 1 row affected (0.09 sec) mysql> insert into DemoTable values('David'); Query OK, 1 row affected (0.07 sec) mysql> insert into DemoTable values(''); Query OK, 1 row affected (0.10 sec) mysql> insert into DemoTable values('Mike'); Query OK, 1 row affected (0.15 ... 阅读更多

让我们看看第一个语法，它在 MAX() 中使用 DISTINCT：select max(DISTINCT yourColumnName) from yourTableName;第二个语法如下所示。它没有使用 DISTINCT：select max( yourColumnName) from yourTableName;注意 - 以上两个查询无论是否使用 DISTINCT 关键字，结果都相同。MySQL 内部将 MAX(yourColumnName) 转换为 DISTINCT 关键字。让我们现在来看一个例子并创建一个表：mysql> create table DemoTable (    Number int ); Query OK, 0 rows affected (1.50 sec)使用 insert 命令在表中插入一些记录：mysql> insert into DemoTable values(80); Query OK, 1 row affected (0.09 sec) mysql> insert into ... 阅读更多

为此，使用 COALESCE()。让我们实现一个存储过程来检查局部变量是否为 null：mysql> DELIMITER // mysql> CREATE PROCEDURE local_VariableDemo()    BEGIN    DECLARE value1 int;    DECLARE value2 int;    select value1, value2;    select    concat('After checking local variable is null the sum is = ', COALESCE(value1, 0)+COALESCE(value2, 0)); END // Query OK, 0 rows affected (0.19 sec) mysql> DELIMITER ;使用 CALL 命令调用存储过程：mysql> call local_VariableDemo();这将产生以下输出：+--------+--------+ | value1 | value2 | +--------+--------+ | NULL | NULL | +--------+--------+ 1 ... 阅读更多

对于基于条件的更新，请使用 UPDATE 和 IF()。让我们首先创建一个表：mysql> create table DemoTable (    Value int ); Query OK, 0 rows affected (0.47 sec)使用 insert 命令在表中插入一些记录：mysql> insert into DemoTable values(10); Query OK, 1 row affected (0.08 sec) mysql> insert into DemoTable values(20); Query OK, 1 row affected (0.10 sec) mysql> insert into DemoTable values(10); Query OK, 1 row affected (0.10 sec) mysql> insert into DemoTable values(20); Query OK, 1 row affected (0.11 sec) mysql> insert into DemoTable values(30); Query OK, 1 row affected (0.10 sec)显示所有记录... 阅读更多

让我们首先创建一个表：mysql> create table DemoTable (    `Values` varchar(50) ); Query OK, 0 rows affected (1.15 sec)使用 insert 命令在表中插入一些记录：mysql> insert into DemoTable values('23, 45, 78, 56'); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable values('384, 476, 7456'); Query OK, 1 row affected (0.12 sec) mysql> insert into DemoTable values('68, 8, 88, 89'); Query OK, 1 row affected (0.10 sec) mysql> insert into DemoTable values('78, 80, 84'); Query OK, 1 row affected (0.19 sec)使用 select 语句显示表中的所有记录：mysql> select *from DemoTable;这... 阅读更多

让我们首先创建一个表：mysql> create table DemoTable (    EmployeeId int NOT NULL AUTO_INCREMENT PRIMARY KEY,    EmployeeName varchar(40),    isMarried boolean ); Query OK, 0 rows affected (0.57 sec)使用 insert 命令在表中插入一些记录：mysql> insert into DemoTable(EmployeeName, isMarried) values('Chris', true); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable(EmployeeName, isMarried) values('Robert', false); Query OK, 1 row affected (0.15 sec) mysql> insert into DemoTable(EmployeeName, isMarried) values('Mike', false); Query OK, 1 row affected (0.21 sec) mysql> insert into DemoTable(EmployeeName, isMarried) values('Bob', true); Query OK, 1 row affected (0.07 sec) mysql> insert into ... 阅读更多

5 rows in set (0.00 sec)