SQL must know

1. Understanding SQL

The simplest way is to think of the database as a file cabinet . This file cabinet is a physical location for storing data .

When you put the information in the cabinet, you create the file in the cabinet and then put the relevant information in a specific file . This file is called a table .

　　table: A structured list of data of a particular type .

　　The data stored in the table is the same type of data or manifest .

　　Schema: Information about the layout and characteristics of databases and tables .

　　The table is made up of columns.

　　column: A field in the table. All tables are made up of one or more columns .

　　each column in the database has a corresponding data type .

　　data type: The type of data that is allowed. Each table column has a corresponding data type that restricts (or allows) the data stored in that column .

　　The data in the table is stored on a row, and each saved record is stored in its own row .

　　row: A record in the table .

　　Each row in the table should have a column (or columns) to uniquely identify itself.

primary KEY (primary key): A column (or set of columns) whose value uniquely identifies each row in the table .

2. Retrieving data

1) retrieving a single column

　　using Select to retrieve table data, you must give at least two messages---what you want to choose and where to choose from .

SELECT Prod_name  from Products;

The preceding statement uses the SELECT statement to retrieve a column named Pro_name from the Products table.

2) Retrieving multiple columns

SELECT Prod_id,prod_name,prod_price  from Products;

3) Retrieving all columns

SELECT *  from Products;

4) Retrieving different values

Use the DISTINCT keyword, which instructs the database to return only different values

SELECT DISTINCT vend_id  from Products;

　　

5) Limit Results

SELECT Prod_name  from  5;

The preceding code uses the SELECT statement to retrieve a single column of data. LIMIT 5 Instructs the DBMS, such as MySQL, to return no more than 5 rows of data.

To get back 5 rows of data, you need to specify where to start and the number of rows to retrieve:

SELECT Prod_name  from  55;

The LIMIT 5 OFFSET 5 instructs the DBMS such as MySQL to return 5 rows of data from line 5th. The first number is the number of rows retrieved, and the second number refers to where to start.

　　Note: The first row to be retrieved is the No. 0 row, not the 1th row. Therefore, the LIMIT 1 OFFSET 1 retrieves the 2nd row instead of the first row .

　　

SQL must know