SQL learning

Learning goals ：

Have a good command of SQL sentence

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Learning content ：

Tips ： Here you can add what you want to learn
1、 SQL Language is basically used
2、 Database related knowledge

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Learning notes

The first lesson understand SQL

One 、 database ： A database is a collection of data stored in an organized way . The simplest way to understand it is to think of the database as a file cabinet .
Two 、1、 The concept of table ： If the database is a file cabinet , That table is a document . Table is a kind of Structured files , Can be used to store a specific type of data .
2、 Features of the watch ：
<1> The data stored in the table is the same type of data or list .
<2> Every table in the database has a name to identify itself . This name is unique , That is, no other table in the database has the same name .
Particular attention ： Make the table unique , In fact, it is a combination of database name and table name , That is to say, the same table name can be used in different databases .
3、 ... and 、 Pattern (schema)： Information about the layout and properties of databases and tables .
Four 、 Columns and data types
Table consists of columns . Columns store information about a part of a table .
1、 Definition of columns ： A column is a field of a table . All tables consist of one or more columns .
Each column has its own data type .
2、 Data type definition ： What types of data are allowed . Each table column has its own data type , It limits the data stored in this column .
5、 ... and 、 That's ok
The data in the table is stored by row , Each record saved is in its own line .
Definition of line ： A record in the table .
6、 ... and 、 Primary key
1、 Primary key definition ： Every row in the table has a column （ Or several columns ） You can uniquely identify yourself .
2、 Primary key properties ：

• Any two rows do not have the same primary key value
• Each row must have a primary key value （ The primary key value cannot be null NULL）
• The value in the primary key column is not allowed to be modified or updated
• Primary key values cannot be reused （ If you delete from the list , Its primary key cannot be paid to new lines in the future ）

The second lesson Retrieving data

One 、SELECT sentence
1、 keyword ： As SQL Reserved words for components . Keywords cannot be used as the names of tables or columns
Common keywords are ：
<1>distinct

 Display the product name without duplicate records , List of commodity prices and commodity categories 
select distinct ware_name,price from t_ware;

<2> Use calculated Columns

 Check all commodity price increases 20% After the price of 
select ware_id,ware_name,price*1.2 from t_ware'

<3> Alias of column

a)  Don't use as
select ware_id,ware_name,price*1.2 price_raise from t_ware;
b) Use as
select ware_id,ware_name,price*1.2 price_raise from t_ware;

2、 Retrieve a single column
The next operation will be in mysql Proceed under , This is a sample table
link ：https://pan.baidu.com/s/15LFHKT-9jG4ImhtMSqVC9Q
Extraction code ：qa2v
Input

SELECT prod_name
FROM Products;

This is from Products Search for a table named prode_name The column of . The column names of the required columns are written in SELECT After keyword ,FROM Keywords indicate which table to retrieve data from .
Here we need to pay attention to SQL Statement is case insensitive . But according to habit SQL Capitalize keywords , Lower case the column name and table name
3、 Retrieve multiple columns
To retrieve multiple columns from a table , Still use the same SELECT sentence . The only difference is that it has to be SELECT Multiple column names are given after the keyword , Column names must be separated by commas . Be careful ： The last column name cannot be added with a comma .
4、 Retrieve all columns
In addition to specifying the required columns （ As mentioned above , One or more columns ）,SELECT Statement can also retrieve all columns without having to list them one by one . Use an asterisk in the position of the actual column name (*) Wildcards can do this . If given a wildcard (*), Then all columns in the table . Look at code

SELECT *
FROM Products;

5、 Retrieve different values
Now I just want to search Products Suppliers of all products in the table ID.

SELECT vend_id
FROM Products;

6、 Restriction result
SELECT Statement returns all matching rows in the specified table , Probably every line . If you want to return a row or a certain number of rows , Need to use limit.
for example , Now I want to limit the return of five lines

SELECT prod_name
FROM Products
LIMIT 5;