MySQL data query (subtotal and sorting)

1, GROUP BY clause

The GROUP BY clause is primarily used to group rows based on fields.For example, all rows in the student base table are grouped according to the major the student is studying, resulting in a group of students for each major.

Syntax format: GROUP BY[column name] [ ASC | DESC ],...[ WITH ROLLUP ]

GROUP BY can be used to group by one or more columns or by expressions, and is often used with aggregate functions.

Example: output the book category name in the Book table

SELECT Book CategoryFROM BookGROUP BY book category;

Example: Count the inventory of various books in the Book table by book category

SELECT book category,COUNT(*)AS 'stock number'FROM BookGROUP BY book category;

Use the GROUP BY clause with the ROLLUP operator to specify that the result set does not contain normal rows provided by GROUP BY, but also includes summary rows.

Example: Count the inventory of various books in the Book table by book category and publisher, and make a subtotal by category

SELECT book category, publisher, Sum (quantity) AS 'stock number'FROM BookGROUP BY Book Category, PublisherWITH ROLLUP;

2. HAVING clause

The purpose of using the HAVING clause is similar to the WHERE clause, except that the WHERE clause is used to select rows after the FROM clause, and the HAVING clause is used to select rows after the GROUP BY clause.

Syntax format: HAVING condition

The SQL standard requires that HAVING must refer to columns in the GROUP BY clause or columns used in aggregate functions.However, MySQL supports extensions to the nature of this work and allows HAVING to reference columns in the SELECT list and columns in outer subqueries.

Example: Find the ID number and average number of books ordered by each member in the Sell table whose average number of books is more than 10.

SELECT ID number, AVG (number of books ordered) AS 'average number of books ordered'FROM SellGROUP BY ID numberHAVING AVG(Number of copies)>10;

3. ORDER BY clause

In a SELECT statement, if the ORDER BY clause is not used, the order of the rows in the result is unpredictable.Using the ORDER BY clause ensures that the rows in the result are in a certain order.

Syntax format: ORDER BY{column name | expression | column number}[ ASC | DESC ],...

The keyword ASC means ascending order, DESC means descending order, the system default value is ASC

Example: Sort the records in the Book table by publication time

SELECT *FROM BookORDER BY Publication date;

Example: Arrange the records in the Sell table from high to bottom according to the number of books ordered

SELECT *FROM SellORDER BY DESC;

4. LIMIT clause

The LIMIT clause is the last clause of the SELECT statement and is mainly used to limit the number of rows returned by the SELECT statement

Syntax format: LIMIT{[offset,]lines|linesOFFSEToffset}

For example, LIMIT 5 means returning the first 5 rows in the result set of the SELECT statement, while LIMIT 3,5 means returning 5 rows starting from the fourth row.

Example: Find the information of the top 5 registered members in the Members table

SELECT * FROM MembersORDER BY registration timeLIMIT 5;

When the initialization is not from scratch, use two parameters LIMIT offset, the number of lines, it is worth noting that The offset of the initial line is 0 instead of 1.

Example: Find 5 records starting from the 4th record in the Book table

SELECT *FROM BookORDER BY student numberLIMIT 3,5;