MOOC and Dechen database course self-use notes _4_SQL overview and simple SQL statements

SQL

1. SQL overview

A database language that integrates DDL, DML, and DCL

1. Leading words in DDL sentences

• Create
• Alter (modified)
• Drop

Mode definition and deletion, including the definition of Database, Table, View, Index, integrity constraints, etc., as well as the definition of objects (RowType row objects, Type column objects)

2. DML sentence guide words

• Insert
• Delete
• Update
• Select

1. Various methods of update and retrieval operations, such as direct input of records, input from other Tables (created by SubQuery)
2. Retrieval of various complex conditions, such as connection search, fuzzy search, group search, nested search, etc.
3. Various aggregation operations, such as average, sum, etc., group aggregation, group filtering, etc.

3. Leading words in DCL sentences

• Grant
• Revoke (Revocation of authorization)

Security control: authorization and revocation of authorization

2. SQL statement

Include two things:

1. Define the database and table (using DDL)
2. Append tuples to the table (using DML)

1. DDL（Data Definition Language）

DDL is usually used by DBA, and it is also used by application programmers after DBA authorization

DDL basic sentences

• Create a database:Create Database
• Create the Table in the DB (define the relational schema):Create Table
• Define the constraints of the Table and its various attributes (define integrity constraints)
• Define View (define external mode and EC image)
• Define Index, Tablespace... etc. (define physical storage parameters)
• Revocation and amendment of the above definitions

DDL example

Create DB

Create Database 数据库名;

Create Table

Create Table 表名(列名 数据类型[Primary key | Unique][Not null][,列名 数据类型[Not null],...]);

Primary key: Primary key constraint. Each table can only create one primary key constraint
Unique: unique constraint (ie, candidate key). There can be multiple uniqueness constraints
Not null: non-null constraints. Means that the column does not allow null values ​​to appear

2. DML（Data Manipulation Language）

DML is usually used by users or application programmers to access authorized databases

DML basic sentences

• Append a new tuple to Table:Insert
• Modify some attributes in some tuples of Table:Update
• Delete some tuples in Table:Delete
• Retrieve various conditions for the data in the Table:Select

DML example

Add tuples to the table. If the column name is omitted, it must be in the same order as the defined or stored column name. If the column name is not omitted, it must be in the same order as listed in the statement.

Insert into 表名[(列名[,列名]...)] 
Values(值[, 值],...);

3. Revision and cancellation of database definitions SQL statements

1. Revise the basic table

Alter Table 表名 
[Add [[列名 数据类型],...]]
[Drop 完整性约束名]
[Modify [[列名 数据类型],...]];

• Add : Add a new column
• Drop : Remove integrity constraints
• Modify : Modify column definition

Example: Delete the constraint that the student's name must be unique

Alter Table Student
Drop Unique(Sname);

2. Delete table

Drop Table 表名;

3. Delete the database

Drop Database 数据库名;

4. Specify/Close the current database

use|close 数据库名;

4. Basic query SQL statement

1. Simple query

Equivalent to ${\prod }_{Columnname,...,The\; columnname}\left(d_{Subjectsearchbarmember}\right(Tablename\left)\right)$ , pay attention to operator precedence and() () forsearch conditions$\left(\right)$ Use

Select 列名[[, 列名]...] 
From 表名
[Where 检索条件];

2. Use the query results Distincetto duplicate records

Select Distinct 列名[[, 列名]...] 
From 表名
[Where 检索条件];

3. query results using Order Bysorting

Sort according to the specified column name, ascor omit it and write it in ascending order, descas descending

Select 列名[[, 列名]...] 
From 表名
[Where 检索条件]
Order By 列名 [asc|desc];

4. Fuzzy query

Use Likeor Not Likekeyword

Select 列名[[, 列名]...] 
From 表名
Where [Not] Like "匹配字符串";

Matching rules:

• % : Match 0 or more characters
• _ : Match any single character
• \ : Escape characters, treat some special characters as ordinary characters

5. Connect query

Equivalent to ${\prod }_{Columnname,...,The\; columnname}\left(d_{Subjectsearchbarmember}\right(Tablename1\times Tablename2\times ...))$

Select 列名[[, 列名]...] 
From 表名1, 表名2,...
Where 检索条件;

When a multi-table join two tables same attribute name, you need to use 表名.属性名way to define which attribute table belongs to the

6. asDuplicate name processing

Select 列名 as 列别名 [[, 列名 as 列别名]...] 
From 表名1 as 表别名1, 表名2 as 表别名2,...
Where 检索条件;

as Can be omitted

7. and Insert, Deleteand Updatenested sub queries binding

Batch data insertion operation

Insert into 表名[(列名[, 列名]...)]
子查询;

Delete certain query data

Delete From 表名 
Where 属性名 in 
(子查询);

Update the specified data

Update 表名
Set 列名 = 表达式|(子查询)
	[[, 列名 = 表达式|(子查询)]...]
[Where 条件表达式];