C language and SQL Server database technology

Join query and group query

One 、 Use Group  By Grouping

1. The essence of grouping is classification , Then merge the same items

2. Grouping is to enable the aggregation function in the next step to make statistics on data of different classifications

3. Check the boys , Data volume of girls

4. Query the number of books in each category

5. Be careful ： Use grouped query statements , The data that can appear in the result column needs to meet one of the following conditions ：

              a、 Columns participating in grouping

              b、 Columns that use aggregate functions

Two 、 Use having Statement filter grouping results

1. Query the number of books in each category , Then sort in descending order according to the number of books

2.order  by Put it on group  by Behind

3. Query the number of books in each category , Filter out the classified data with only one book

4.having Statement is used to filter the grouped results

5. The execution order of the three statements is ：

            where    --》  group  by    --》    having

6. Query books whose author is not empty , What is the number of books corresponding to each category , And filter out the classified data with only one book

3、 ... and 、 Inner join query

1. To prevent data redundancy , Our data is usually stored in different tables according to categories

2. If you want to view the complete data , You need to query data from multiple tables to combine them into a complete query result

3. This requires the use of multi table associated query , The multi table associated query revolves around the associated columns （ Foreign keys ） On going

4. Relational query ：

              a、 Inner join   inner  join

              b、 External connection   outer  join

5. Inner join query ：

              a、 Use inner  join  ...  on  ...  keyword , among inner It can be omitted

              b、 Inner join query is the most commonly used in join query

              c、on  What follows is the correspondence between the two tables connected , Connection conditions

              d、 Only the data with corresponding relationship in both tables will be displayed

              e、as It can not only be used to alias columns , You can also alias the table

              f、 Table name .*  Indicates that all columns of this table are displayed

              g、 Can pass where The condition specifies the writing method of implicit inner connection

              h、 The order of the tables in the inline connection only indicates which table initiates the query , It does not affect the number of rows in the query result , That is, two tables are equal

              i、 You can connect multiple tables at one time , For example, student table 、 Chart of subjects 、 Joint query of the third form of the transcript

Four 、 External connection query

1. Inner join query two tables are equal , Only data that meets the connection conditions can be displayed

2. The query method opposite to inner join is called outer join query , It can be used to filter records that do not meet the connection conditions

3. Syntax of external connection query ：

left outer join ... on ...
right outer join ... on ...

4. Features of external connection query ：

             a、 The tables participating in the connection in the external connection query can be divided into master and slave

             b、 Match the data columns of the slave table with each row of the master table

             c、 The qualified data is directly returned to the result set

             d、 Unqualified records are filled in null Still return to the result set

             e、 Whether the records of the main table match in the sub table , Can be guaranteed to appear in the query results

             f、 The left table is the main table and becomes the left outer connection , The right table with the main table becomes the right outer connection

4. Query the book classification information without a book record