ddl | 易学教程

What is DDL and DML? [closed]

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问题 I have heard the terms DDL and DML in reference to databases, but I don\'t understand what they are. What are they and how do they relate to SQL? 回答1: DDL is Data Definition Language : it is used to define data structures . For example, with SQL, it would be instructions such as create table , alter table , ... DML is Data Manipulation Language : it is used to manipulate data itself . For example, with SQL, it would be instructions such as insert , update , delete , ... 回答2: More information

How does spring.jpa.hibernate.ddl-auto property exactly work in Spring?

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问题 I was working on my Spring boot app project and noticed that, sometimes there is a connection time out error to my Database on another server(SQL Server). This happens specially when I try to do some script migration with FlyWay but it works after several tries. Then I noticed that I didn\'t specify spring.jpa.hibernate.ddl-auto in my properties file. I did some research and found that it is recommended to add spring.jpa.hibernate.ddl-auto= create-drop in development. And change it to: spring

Sqlite primary key on multiple columns

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问题 What is the syntax for specifying a primary key on more than 1 column in SQLITE ? 回答1: According to the documentation, it's CREATE TABLE something ( column1, column2, column3, PRIMARY KEY (column1, column2) ); 回答2: CREATE TABLE something ( column1 INTEGER NOT NULL, column2 INTEGER NOT NULL, value, PRIMARY KEY ( column1, column2) ); 回答3: Yes. But remember that such primary key allow NULL values in both columns multiple times. Create a table as such: sqlite> CREATE TABLE something ( column1,

Safely rename tables using serial primary key columns

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问题 I know that PostgreSQL tables that use a SERIAL primary key end up with an implicit index, sequence and constraint being created by PostgreSQL. The question is how to rename these implicit objects when the table is renamed. Below is my attempt at figuring this out with specific questions at the end. Given a table such as: CREATE TABLE foo ( pkey SERIAL PRIMARY KEY, value INTEGER ); Postgres outputs: NOTICE: CREATE TABLE will create implicit sequence \"foo_pkey_seq\" for serial column \"foo

Mysql学习笔记（2）--DDL语句

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mysql学习笔记（1） https://blog.csdn.net/Fhujinwu/article/details/81517046 1、SQL语句主要划分为三个类别： ①DDL语句：数据定义语言，这些语句定义了不同的数据段、数据库、表‘、列、索引等数据库对象，常用的语句关键字主要包括create、drop、alter等； ②DML语句：用于添加、删除、更新和查询数据库记录，并检查数据完整性，常用的语句关键字主要包括insert、delete、select等； ③DCL语句：数据控制语句，用于控制不同数据段直接的许可和访问级别的语句。这些语句定义了数据库、表、字段、用户的访问权限和安全级别。主要语句关键字包括grant、revoke等； 2、在mysql>提示符后面输入所要执行的SQL语句，每个SQL语句以分号（；）或者“\g”结束，按回车键执行； 3、创建一个新的数据库test1： create database test1; 显示系统中已经存在的所有数据库：show databases；查看已存在数据库中的所有数据表： use test (回车） show tables；（回车）删除已存在的数据库：drop databses test； 4、在数据库中创建一张表的步骤：首先选定数据库 use databasename，回车，之后输入信息，格式如下所示： CREATE

MySQL学习笔记--DQL、DML、DDL、TCL语句，约束（主键、外键）

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MySQL 使用工具 Windows自带的命令行窗口 MySQL Front 一、MySQL常用数据类型 1. char 占用字节数 char(n) 定长字符串，存储控件大小固定如：char(2) 2. varchar 占用字节数 varchar(n) 变长字符串，存储空间等于实际数据空间只包含英文字符的字符串 3. int 占用字节数 4个字节整型比如自增ID和表示数量 4. bigint 占用字节数 8个字节长整型数量比较大的自增ID float（数值型）、double（数值型）、date（日期时间，8个字节）、BLOB（二进制大对象）、CLOB（字符大对象）… 二、SQL语句分类 1. DQL语句—数据查询语言(select) 1.1 简单查询查询单个字段 select 字段名 from 表名; 注：所有符号为英文下符合查询多个字段 select 字段名,字段名 from 表名; 查询所有字段 select * from 表名; 1.2 练习（计算每位员工的年薪） select ename,sal*12 from emp; //进一步修改一下查询年薪后的字段名 select ename,sal*12 as yearsal from emp; 1.3 条件查询注意：条件查询需要用到where语句，where必须放到from语句表的和面；等号（=）操作符 /

How to convert primary key from integer to serial?

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问题 In a Postgres 9.3 table I have an integer as primary key with automatic sequence to increment, but I have reached the maximum for integer . How to convert it from integer to serial ? I tried: ALTER TABLE my_table ALTER COLUMN id SET DATA TYPE bigint; But the same does not work with the data type serial instead of bigint . Seems like I cannot convert to serial ? 回答1: serial is a pseudo data type, not an actual data type. It's an integer underneath with some additional DDL commands executed

Can a number be used to name a MySQL table column?

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问题 I have a table that has column names like 25, 50, 100, etc.. When trying to update the table I get an error, no matter how I do it UPDATE table SET \'25\'=\'100\' WHERE id = \'1\' I have tried quoting and backticking every which way but without success. The error is always along the lines of: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near \'\'25\'=100 WHERE id=1\' at line 1 If I change the column name to

Is it possible to roll back CREATE TABLE and ALTER TABLE statements in major SQL databases?

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问题 I am working on a program that issues DDL. I would like to know whether CREATE TABLE and similar DDL can be rolled back in Postgres MySQL SQLite et al Describe how each database handles transactions with DDL. 回答1: http://wiki.postgresql.org/wiki/Transactional_DDL_in_PostgreSQL:_A_Competitive_Analysis provides an overview of this issue from PostgreSQL's perspective. Is DDL transactional according to this document? PostgreSQL - yes MySQL - no; DDL causes an implicit commit Oracle Database 11g

How do you like your primary keys? [closed]

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问题 As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance. Closed 7 years ago . In a fairly animated discussion in my team I was made to think what most people like as primary keys. We had the following groups- Int