SELECT in mysql language

SELECT

Provide data query function
Syntax: SELECT [ALL | DISTINCT | DISTINCTROW ] [HIGH_PRIORITY] [STRAIGHT_JOIN] [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT] [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS] select_expr [, select_expr ...] [FROM table_references [WHERE where_condition] [GROUP BY {col_name | expr | position} [ASC | DESC], ... [WITH ROLLUP]] [HAVING where_condition] [ORDER BY {col_name | expr | position} [ASC | DESC], ...] [LIMIT {[offset,] row_count | row_count OFFSET offset}] [PROCEDURE procedure_name(argument_list)] [INTO OUTFILE 'file_name' [CHARACTER SET charset_name] export_options | INTO DUMPFILE 'file_name' | INTO var_name [, var_name]] [FOR UPDATE | LOCK IN SHARE MODE]]

1) Query all information in the form

MariaDB [hellodb]> SELECT * FROM students;

2) Query a specific field in the form

MariaDB [hellodb]> SELECT user,host FROM mysql.user;

3) Field display alias function

MariaDB [hellodb]> select user as 名字,host as 主机名 FROM mysql.user;
+--------+-----------+
| 名字   | 主机名    |
+--------+-----------+
| root   | 127.0.0.1 |
| root   | ::1       |
|        | ansible   |
| root   | ansible   |
|        | localhost |
| root   | localhost |
+--------+-----------+

4) WHERE filter condition query.

name	Operator
Arithmetic comparison operator	+, -, *, /, %
Comparison operator	=, !=, <>, <=>, >, >=, <, <=
Judge null	IS NULL
Judge not empty	IS NOT NULL
Which fields are included	IN (element1, element2, …)
A certain range	BETWEEN min_num AND max_num

MariaDB [ydong]> SELECT * FROM students WHERE id!=1;
+----+----------+-------+
| id | name     | phone |
+----+----------+-------+
|  2 | xiaohong |   456 |
|  3 | xiaobai  |  NULL |
+----+----------+-------+
2 rows in set (0.02 sec)

MariaDB [ydong]> SELECT * FROM students WHERE id BETWEEN 1 AND 2;
+----+----------+-------+
| id | name     | phone |
+----+----------+-------+
|  1 | xiaoming |   123 |
|  2 | xiaohong |   456 |
+----+----------+-------+
2 rows in set (0.00 sec)


MariaDB [ydong]> SELECT * FROM students WHERE phone IS NULL;
+----+---------+-------+
| id | name    | phone |
+----+---------+-------+
|  3 | xiaobai |  NULL |
+----+---------+-------+
1 row in set (0.00 sec)


MariaDB [ydong]> SELECT * FROM students WHERE name IN ('xiaobai');
+----+---------+-------+
| id | name    | phone |
+----+---------+-------+
|  3 | xiaobai |  NULL |
+----+---------+-------+
1 row in set (0.01 sec)

5) like, query the fields that meet the conditions defined

• %: any character of any length
• _: any single character

MariaDB [ydong]> SELECT * FROM students WHERE name LIKE '%g%';
+----+----------+-------+
| id | name     | phone |
+----+----------+-------+
|  1 | xiaoming |   123 |
|  2 | xiaohong |   456 |
+----+----------+-------+
2 rows in set (0.01 sec)

6) GROUP: According to the specified conditions, the query results are "grouped" for "aggregation" operation

• avg(): find the average
• max(): Maximum
• min(): minimum
• count(): count
• sum(): sum

MariaDB [ydong]> select sex,avg(score) from students group by sex; 
+------+------------+
| sex  | avg(score) |
+------+------------+
| f    |         90 |
| m    |       87.5 |
+------+------------+
2 rows in set (0.01 sec)
根据性别取成绩平均值

7. HAVING: Specify the filter condition for the result of the grouping aggregation operation, as long as there is a grouping, you must use having

MariaDB [ydong]> SELECT sex,avg(score) from students group by sex having avg(score) > 80 ;
+------+-------------------+
| sex  | avg(score)        |
+------+-------------------+
| f    | 86.66666666666667 |
+------+-------------------+
1 row in set (0.01 sec)

8) ORDER BY: Sort the query results according to the specified field

MariaDB [ydong]> SELECT name,score FROM students ORDER BY score DESC;
+-----------+-------+
| name      | score |
+-----------+-------+
| xiaobai   | 95    |
| xiaohong  | 90    |
| xiaoming  | 80    |
| xiaolan   | 70    |
| xiaocheng | 60    |
| xiaohuang | 100   |
+-----------+-------+
6 rows in set (0.00 sec)
DESC 顺序排列


MariaDB [ydong]> SELECT name,score FROM students ORDER BY score ASC;
+-----------+-------+
| name      | score |
+-----------+-------+
| xiaohuang | 100   |
| xiaocheng | 60    |
| xiaolan   | 70    |
| xiaoming  | 80    |
| xiaohong  | 90    |
| xiaobai   | 95    |
+-----------+-------+
6 rows in set (0.02 sec)
逆序排列




MariaDB [ydong]> SELECT name,phone FROM students ORDER BY -phone ASC;
+-----------+-------+
| name      | phone |
+-----------+-------+
| xiaobai   |  NULL |
| xiaohuang |  NULL |
| xiaolan   |  NULL |
| xiaocheng |  NULL |
| xiaohong  |   456 |
| xiaoming  |   123 |
+-----------+-------+
6 rows in set (0.01 sec)
-phone意思是将null值按照逆序ASC还是顺序DESC排列，仅对数字有效

9) LIMIT [[offset,]row_count]: limit the number of output rows of the query results

MariaDB [ydong]> SELECT name,phone FROM students ORDER BY -phone ASC LIMIT 2,4;
+-----------+-------+
| name      | phone |
+-----------+-------+
| xiaolan   |  NULL |
| xiaocheng |  NULL |
| xiaohong  |   456 |
| xiaoming  |   123 |
+-----------+-------+
4 rows in set (0.01 sec)
意思是从第三行往下数4行。

Multi-table query

1) The left join will read all the data of A, even if there is no corresponding data
in table B. We use students as table A and table B as teachers table.

MariaDB [hellodb]> select stu.Name as stu_name ,t.Name as teacher_name  from students as stu left join teachers as t on stu.TeacherID=t.TID;

+---------------+---------------+
| stu_name      | teacher_name  |
+---------------+---------------+
| Shi Zhongyu   | Miejue Shitai |
| Shi Potian    | NULL          |
| Xie Yanke     | NULL          |
| Ding Dian     | Lin Chaoying  |
| Yu Yutong     | Song Jiang    |
...

Inner join, take the intersection of two tables

MariaDB [hellodb]> SELECT stu.name as 学生名字,t.name as 教师名字 FROM students as stu inner join teachers as t on stu.TeacherID=t.TID;
+--------------+---------------+
| 学生名字     | 教师名字      |
+--------------+---------------+
| Yu Yutong    | Song Jiang    |
| Shi Zhongyu  | Miejue Shitai |
| Ding Dian    | Lin Chaoying  |
+--------------+---------------+
3 rows in set (0.02 sec)

Right connection, opposite to left connection

MariaDB [hellodb]> SELECT t.Name as 教师名字, stu.Name as 学生名字 FROM students as stu right join teachers as t on stu.TeacherID=t.TID;
+---------------+--------------+
| 教师名字      | 学生名字     |
+---------------+--------------+
| Miejue Shitai | Shi Zhongyu  |
| Lin Chaoying  | Ding Dian    |
| Song Jiang    | Yu Yutong    |
| Zhang Sanfeng | NULL         |
+---------------+--------------+
4 rows in set (0.01 sec)

4）

A table and B table are displayed at the same time

Two tables are displayed at the same time using union

MariaDB [hellodb]> SELECT Stuid,Name from students
    -> union
    -> SELECT Tid,Name from teachers;

5）

Remove the field of A after the intersection of A and B

MariaDB [hellodb]> SELECT stu.Name,t.Name AS teacher_name FROM students AS stu LEFT JOIN teachers AS t on stu.TeacherID=t.TID WHERE t.name is null;

Subquery

In the query statement in the nested query statement, the performance is generally poor

Used for subqueries in where clauses

MariaDB [hellodb]> SELECT Name,Age FROM students WHERE Age > (SELECT Avg(Age) FROM students);
+--------------+-----+
| Name         | Age |
+--------------+-----+
| Xie Yanke    |  53 |
| Ding Dian    |  32 |
| Shi Qing     |  46 |
| Tian Boguang |  33 |
| Sun Dasheng  | 100 |
+--------------+-----+
5 rows in set (0.05 sec)

Subqueries used in IN

MariaDB [hellodb]> SELECT Name,Age FROM students WHERE Age IN (SELECT Age FROM teachers);

Used for subqueries in FROM clause

MariaDB [hellodb]> SELECT * FROM (SELECT ClassID,AVG(Age) AS aage FROM students GROUP BY ClassID HAVING ClassID IS NOT NULL) AS s WHERE s.aage < 30;
+---------+---------+
| ClassID | aage    |
+---------+---------+
|       1 | 20.5000 |
|       3 | 20.2500 |
|       4 | 24.7500 |
|       6 | 20.7500 |
|       7 | 19.6667 |
+---------+---------+
5 rows in set (0.00 sec)