ALTER TABLE 表名 ADD COLUMN IF NOT EXISTS 列名 列数据类型 DEFAULT 默认值; IF NOT EXISTS这个参数用法就是让查询时若该列不存在,则自动添加,若存在则什么也不做。因此,运用此语句能够达到自动添加列的目的。 一个具体的例子如下所示: ALTER TABLE user ADD COLUMN IF NOT EXISTS mobile VARCHAR(50) DEFAUL...
DEFAULT CHARSET=utf8mb4; -- 查看表结构 DESCRIBE students; -- 或 SHOW COLUMNS FROM students; -- 修改表结构 ALTER TABLE students ADD COLUMN email VARCHAR(100); ALTER TABLE students MODIFY COLUMN name VARCHAR(100); ALTER TABLE students DROP COLUMN email; -- 删除表 DROP TABLE [IF EXISTS] ...
0 运行 AI代码解释 mysql> create table if not existstest_key( -> id int unsigned primary key comment '这个是学生的学号', -> name varchar(20) not null -> ); Query OK, 0 rows affected (0.03 sec) mysql> desc test_key; +---+---+---+---+---+---+ | Field | Type | Null...
下面是删除表字段的示例代码: ALTERTABLEemployeesDROPCOLUMNsalary; 1. 2. 以上代码将删除employees表中的salary字段。 错误提示:check that column/key exists 在我们执行删除表字段的操作时,可能会遇到以下错误提示:check that column/key exists。这个错误提示意味着MySQL无法找到要删除的字段。造成该错误的原因有几...
In some situations, you want to check whether a column already exists in a table before adding it. However, there is no statement like ADD COLUMN IF NOT EXISTS available. Fortunately, you can get this information from the columns table of the information_schema database as the following ...
typeCast: Determines if column values should be converted to native JavaScript types. (Default: true) queryFormat: A custom query format function. See Custom format. supportBigNumbers: When dealing with big numbers (BIGINT and DECIMAL columns) in the database, you should enable this option (Defa...
add contraint fk_name FOREIGN KEY (column_name) REFERENCES parent_table (column_name); --添加外键 ] ... 例如: 在基本表Student中增加约束条件:男生年龄小于24岁,女生年龄小于22岁 alter table Student add contraint check_Sex check( case when Ssex = '男' then Ssex < 24 when ...
共分为not null, unique, default, primary key, check这几种 null约束 定义:指示某列不能存储null值 在创建表时,就是指定某列不为空,当指定列插入null会报错 -- 重新设置学生表结构 DROP TABLE IF EXISTS student;CREATE TABLE student (id INT NOT NULL ,sn INT ,name VARCHAR ( 20 ),qq_mail VARCHAR...
..)] [table_options] [partition_options] [IGNORE | REPLACE] [AS] query_expression CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name { LIKE old_tbl_name | (LIKE old_tbl_name) } create_definition: { col_name column_definition | {INDEX | KEY} [index_name] [index_type] (key_part,...
In this case, you may be able to improve the performance of your query by examining the WHERE clause to check whether it refers to some column or columns that would be suitable for indexing. If so, create an appropriate index and check the query with EXPLAIN again. See Section 15.1.9, ...