Summary of MySQL basic knowledge points

1. What is a database ？

A database is a collection of data stored in an organized way .

One of the easiest ways to understand a database is to think of it as a file cabinet . This file cabinet is a physical location for storing data , No matter what the data is and how it is organized .

2. The concept of table

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

Key points ： Data stored in a table is a type of data or a list . Never store the list of customers and the list of orders in the same database table . Doing so will make it difficult to retrieve and access later , Two tables should be created , One table per list .

Be careful ： Each table in the database has a name , To identify yourself . This name is unique . You can use the same table name in different databases .

3. Can't use part of DISTINCT

DISTINCT Keyword applies to all columns, not just the columns that precede it . If given SELECT DISTINCT id,money, Unless both columns specified are different , Otherwise all rows will be retrieved .

4.MYSQL 5 Of LIMIT grammar

LIMIT 3,4 The meaning of is not very clear , It is easy to confuse people , It means from the line 3 At the beginning 4 That's ok . But it's easy to remember backwards

For this reason ,MYSQL 5 Support LIMIT Another alternative grammar for .

LIMIT 4 OFFSET 3 It means from the line 3 Began to take 4 That's ok , It's like LIMIT 3,4 equally .

5. Sorting and retrieving data order by

One thing to note is that , It is often necessary to sort data by more than one column . for example , If you want to display a list of employees , You may want to sort by last name and first name （ First, sort by surname , Then sort by first name in each surname ）.

If more than one employee has the same last name , It's easy to use .

To sort by multiple columns , Just specify the column name , Separate column names with commas （ Just like selecting multiple columns ）.

Specify sorting direction ：

      Data sorting is not limited to ascending sorting （ from A To Z）. This is the default sort order , You can also use ORDER BY Clauses in descending order （ from Z To A） Order . For descending sort , Must specify DESC keyword .

6. Mismatch check

Example ： List not 303 Name of the class    SELECT   class_id,name   From   student   WHERE   class_id <> 303

Be careful ： Use != and <> The effect is the same .

7. Order of calculation

WHERE Can contain any number of AND and OR The operator . Allow both to be combined for complex and advanced filtering .

SQL Like most languages, it deals with OR Before the operator , priority AND The operator .

8. Why use IN The operator ？ Advantages as follows ：

a. When using a long list of legal options ,IN Operator syntax is clearer and more intuitive ;

b. In the use of IN when , The order of calculations is easier to manage （ Because fewer operators are used ）;

c.IN Operator general ratio OR Faster operator list execution ;

d.IN The biggest advantage is that it can contain other SELECT sentence , Make it possible to dynamically create WHERE Clause .