Tutorial on the principle and application of database system (050) -- MySQL query (XII): analysis of the execution process of select command

Database system principle and Application Tutorial （050）—— MySQL Inquire about （ Twelve ）：SELECT Analysis of the execution process of the command

SELECT A statement consists of several parts , You need to follow a certain order when writing , Otherwise, a syntax error will be prompted . In the process of execution , Each part has corresponding execution sequence , When using column aliases, you need to pay attention to the execution order of each part , Otherwise, the alias will not be found .

One 、SELECT The writing order of sentences

SELECT The writing order of each component of the sentence is as follows ：

SELECT DISTINCT < Column name or expression list >
FROM <table_name> [INNER|LEFT|RIGHT] JOIN <table_name>
ON < Connection condition >
WHERE < filter >
GROUP BY < Group fields or expressions >
HAVING < Group filter criteria >
WITH ROLLUP
ORDER BY < Sort field or expression >
LIMIT [m,]n

Wrong writing order SELECT The syntax error will be prompted when the statement is executed （ Error code ：1064）.

Two 、SELECT The order in which statements are executed

SELECT The execution order of each component of the statement is as follows ：

--（1） Specify the table used by the query 
FROM <table_name> [INNER|LEFT|RIGHT] JOIN <table_name>
ON < Connection condition >
--（2） Specify filter criteria （WHERE  Can't be used later  SELECT  Alias of the specified column , Because at this time  select  Clause has not been executed ）
WHERE < filter >
--（3） Grouping data 
GROUP BY < Group fields or expressions >
WITH ROLLUP
HAVING < Group filter criteria >
--（4） Generate the columns and expressions that need to be displayed 
SELECT < List of fields or expressions >
--（5） De duplicate data rows 
DISTINCT
--（6） Sort query results 
ORDER BY < Sort field or expression >
LIMIT [m,]n

Instructions for alias names of columns and expressions ：

（1）WHERE Clause cannot use aliases for columns and expressions .

（2）GROUP BY and ORDER BY Clause can use aliases for columns and expressions .

3、 ... and 、 stay SELECT Use variables in statements

1、 stay FROM Assign values to variables in Clause

Throughout the query process ,FROM Clause is executed only once , therefore FROM The operation of assigning a value to a variable in the clause is performed only once .

Verified as follows ：

/* SELECT @var := 0; SELECT s.* FROM student s,(SELECT @var := @var + 1) a; SELECT @var; */
mysql> SELECT @var := 0;
+-----------+
| @var := 0 |
+-----------+
|         0 |
+-----------+
1 row in set (0.00 sec)

mysql> SELECT s.* FROM student s,(SELECT @var := @var + 1) a;
+-------+-----------+--------+---------------------+-------------+-----------+
| s_id  | s_name    | gender | birth               | phone       | addr      |
+-------+-----------+--------+---------------------+-------------+-----------+
| S2011 |  Zhang Xiaogang     |  male      | 1999-12-03 00:00:00 | 13163735775 |  Xinyang City     |
| S2012 |  Liu Xiaoqing     |  Woman      | 1999-10-11 00:00:00 | 13603732255 |  Xinxiang City     |
| S2013 |  Cao mengde     |  male      | 1998-02-13 00:00:00 | 13853735522 |  Zhengzhou city     |
| S2014 |  Liu Yan       |  Woman      | 1998-06-24 00:00:00 | 13623735335 |  Zhengzhou city     |
| S2015 |  Liu Yan       |  Woman      | 1999-07-06 00:00:00 | 13813735225 |  Xinyang City     |
| S2016 |  Liu Ruofei     |  Woman      | 2000-08-31 00:00:00 | 13683735533 |  Kaifeng City     |
| S2021 |  Dong Wenhua     |  Woman      | 2000-07-30 00:00:00 | 13533735564 |  Kaifeng City     |
| S2022 |  Zhou Huajian     |  male      | 1999-05-25 00:00:00 | 13243735578 |  Zhengzhou city     |
| S2023 |  trump     |  male      | 1999-06-21 00:00:00 | 13343735588 |  Xinxiang City     |
| S2024 |  Obama     |  male      | 2000-10-17 00:00:00 | 13843735885 |  Xinyang City     |
| S2025 |  Zhou Jianhua     |  male      | 2000-08-22 00:00:00 | 13788736655 |  Kaifeng City     |
| S2026 |  Zhang Xueyou     |  male      | 1998-07-06 00:00:00 | 13743735566 |  Zhengzhou city     |
| S2031 |  Lee Myung Bak     |  Woman      | 1999-10-26 00:00:00 | 13643732222 |  Zhengzhou city     |
| S2032 |  Vinci     |  male      | 1999-12-31 00:00:00 | 13043731234 |  Zhengzhou city     |
+-------+-----------+--------+---------------------+-------------+-----------+
14 rows in set (0.00 sec)

mysql> SELECT @var;
+------+
| @var |
+------+
|    1 |
+------+
1 row in set (0.00 sec)

2、 stay SELECT Assign values to variables in Clause

Throughout the query process ,SELECT The number of times the clause is executed depends on the number of rows recorded in the query result . therefore , How many rows are there in the query result , be SELECT The operation of assigning values to variables in the clause is performed several times . According to this characteristic , have access to COUNT(1) Count the number of rows in the query result .

Verified as follows ：

/* SELECT s.*,@var := @var + 1 FROM student s,(SELECT @var := 0) a; SELECT @var; */
mysql> SELECT s.*,@var := @var + 1 FROM student s,(SELECT @var := 0) a;
+-------+-----------+--------+---------------------+-------------+-----------+------------------+
| s_id  | s_name    | gender | birth               | phone       | addr      | @var := @var + 1 |
+-------+-----------+--------+---------------------+-------------+-----------+------------------+
| S2011 |  Zhang Xiaogang     |  male      | 1999-12-03 00:00:00 | 13163735775 |  Xinyang City     |                1 |
| S2012 |  Liu Xiaoqing     |  Woman      | 1999-10-11 00:00:00 | 13603732255 |  Xinxiang City     |                2 |
| S2013 |  Cao mengde     |  male      | 1998-02-13 00:00:00 | 13853735522 |  Zhengzhou city     |                3 |
| S2014 |  Liu Yan       |  Woman      | 1998-06-24 00:00:00 | 13623735335 |  Zhengzhou city     |                4 |
| S2015 |  Liu Yan       |  Woman      | 1999-07-06 00:00:00 | 13813735225 |  Xinyang City     |                5 |
| S2016 |  Liu Ruofei     |  Woman      | 2000-08-31 00:00:00 | 13683735533 |  Kaifeng City     |                6 |
| S2021 |  Dong Wenhua     |  Woman      | 2000-07-30 00:00:00 | 13533735564 |  Kaifeng City     |                7 |
| S2022 |  Zhou Huajian     |  male      | 1999-05-25 00:00:00 | 13243735578 |  Zhengzhou city     |                8 |
| S2023 |  trump     |  male      | 1999-06-21 00:00:00 | 13343735588 |  Xinxiang City     |                9 |
| S2024 |  Obama     |  male      | 2000-10-17 00:00:00 | 13843735885 |  Xinyang City     |               10 |
| S2025 |  Zhou Jianhua     |  male      | 2000-08-22 00:00:00 | 13788736655 |  Kaifeng City     |               11 |
| S2026 |  Zhang Xueyou     |  male      | 1998-07-06 00:00:00 | 13743735566 |  Zhengzhou city     |               12 |
| S2031 |  Lee Myung Bak     |  Woman      | 1999-10-26 00:00:00 | 13643732222 |  Zhengzhou city     |               13 |
| S2032 |  Vinci     |  male      | 1999-12-31 00:00:00 | 13043731234 |  Zhengzhou city     |               14 |
+-------+-----------+--------+---------------------+-------------+-----------+------------------+
14 rows in set (0.00 sec)

mysql> SELECT @var;
+------+
| @var |
+------+
|   14 |
+------+
1 row in set (0.00 sec)

3、 Count the number of rows in the query result

mysql> select count(1) from student where addr=' Zhengzhou city ';
+----------+
| count(1) |
+----------+
|        6 |
+----------+
1 row in set (0.04 sec)

mysql> select sum(1) from student where addr=' Zhengzhou city ';
+--------+
| sum(1) |
+--------+
|      6 |
+--------+
1 row in set (0.02 sec)

Four 、 Alias usage rules for columns

1、WHERE Clause cannot use aliases for columns and expressions

/* select s_id,s_name,birth,year(now())-year(birth) age from student where age > 20; */
--  error ： Alias not recognized  age
mysql> select s_id,s_name,birth,year(now())-year(birth) age 
    -> from student where age > 20;
ERROR 1054 (42S22): Unknown column 'age' in 'where clause'

2、GROUP BY and ORDER BY Clause can use aliases for columns and expressions

（1）GROUP BY Option use alias

/* select year(now())-year(birth) age, count(*) from student group by age; */
mysql> select year(now())-year(birth) age, count(*) 
    -> from student group by age;
+------+----------+
| age  | count(*) |
+------+----------+
|   22 |        4 |
|   23 |        7 |
|   24 |        3 |
+------+----------+
3 rows in set (0.00 sec)

（2）ORDER BY Option use alias

/* select s_id,s_name,birth,year(now())-year(birth) age from student order by age desc; */
mysql> select s_id,s_name,birth,year(now())-year(birth) age 
    -> from student order by age desc;
+-------+-----------+---------------------+------+
| s_id  | s_name    | birth               | age  |
+-------+-----------+---------------------+------+
| S2013 |  Cao mengde     | 1998-02-13 00:00:00 |   24 |
| S2014 |  Liu Yan       | 1998-06-24 00:00:00 |   24 |
| S2026 |  Zhang Xueyou     | 1998-07-06 00:00:00 |   24 |
| S2011 |  Zhang Xiaogang     | 1999-12-03 00:00:00 |   23 |
| S2012 |  Liu Xiaoqing     | 1999-10-11 00:00:00 |   23 |
| S2015 |  Liu Yan       | 1999-07-06 00:00:00 |   23 |
| S2022 |  Zhou Huajian     | 1999-05-25 00:00:00 |   23 |
| S2023 |  trump     | 1999-06-21 00:00:00 |   23 |
| S2031 |  Lee Myung Bak     | 1999-10-26 00:00:00 |   23 |
| S2032 |  Vinci     | 1999-12-31 00:00:00 |   23 |
| S2016 |  Liu Ruofei     | 2000-08-31 00:00:00 |   22 |
| S2021 |  Dong Wenhua     | 2000-07-30 00:00:00 |   22 |
| S2024 |  Obama     | 2000-10-17 00:00:00 |   22 |
| S2025 |  Zhou Jianhua     | 2000-08-22 00:00:00 |   22 |
+-------+-----------+---------------------+------+
14 rows in set (0.00 sec)

5、 ... and 、WHERE and GROUP BY … HAVING Clause

When WHERE and GROUP BY … HAVING When clauses are used at the same time , The execution process is as follows ：

（1） perform WHERE Clause to filter the records in the table , Get the row that meets the given condition .

（2） Group the records that meet the query conditions , And then use HAVING Clause to filter groups .

for example ： according to student The table queries the addresses 【 male 】 Number of students , And display more than 2 Human behavior .

The order is as follows ：

/* select addr,count(*) cnt from student where gender = ' male ' group by addr having cnt>2; */

mysql> select addr,count(*) cnt from student
    -> where gender = ' male '
    -> group by addr having cnt>2;
+-----------+-----+
| addr      | cnt |
+-----------+-----+
|  Zhengzhou city     |   4 |
+-----------+-----+
1 row in set (0.00 sec)

The above query can be executed step by step ：

（1） Inquire about student In the table 【 male 】 Health information

mysql> select * from student where gender = ' male ';
+-------+-----------+--------+---------------------+-------------+-----------+
| s_id  | s_name    | gender | birth               | phone       | addr      |
+-------+-----------+--------+---------------------+-------------+-----------+
| S2011 |  Zhang Xiaogang     |  male      | 1999-12-03 00:00:00 | 13163735775 |  Xinyang City     |
| S2013 |  Cao mengde     |  male      | 1998-02-13 00:00:00 | 13853735522 |  Zhengzhou city     |
| S2022 |  Zhou Huajian     |  male      | 1999-05-25 00:00:00 | 13243735578 |  Zhengzhou city     |
| S2023 |  trump     |  male      | 1999-06-21 00:00:00 | 13343735588 |  Xinxiang City     |
| S2024 |  Obama     |  male      | 2000-10-17 00:00:00 | 13843735885 |  Xinyang City     |
| S2025 |  Zhou Jianhua     |  male      | 2000-08-22 00:00:00 | 13788736655 |  Kaifeng City     |
| S2026 |  Zhang Xueyou     |  male      | 1998-07-06 00:00:00 | 13743735566 |  Zhengzhou city     |
| S2032 |  Vinci     |  male      | 1999-12-31 00:00:00 | 13043731234 |  Zhengzhou city     |
+-------+-----------+--------+---------------------+-------------+-----------+
8 rows in set (0.00 sec)

（2） Press addr Grouping , Count the number of students in each address

/* select addr, count(*) cnt from student where gender = ' male ' group by addr; */

mysql> select addr, count(*) cnt from student where gender = ' male '
    -> group by addr;
+-----------+-----+
| addr      | cnt |
+-----------+-----+
|  Xinyang City     |   2 |
|  Kaifeng City     |   1 |
|  Xinxiang City     |   1 |
|  Zhengzhou city     |   4 |
+-----------+-----+
4 rows in set (0.00 sec)

（3） Use HAVING Clause to filter groups

/* select addr, count(*) cnt from student where gender = ' male ' group by addr having cnt > 2; */
mysql> select addr, count(*) cnt from student where gender = ' male '
    -> group by addr having cnt > 2;
+-----------+-----+
| addr      | cnt |
+-----------+-----+
|  Zhengzhou city     |   4 |
+-----------+-----+
1 row in set (0.00 sec)