[turn to] MySQL operation practice (I): Keywords & functions

MySQL Practical operation （ One ）： keyword & function

The following is the database sqltest Three tables in , Its structure and content （ part ） as follows ：

1. keyword

①EXISTS、NOT EXISTS

EXISTS keyword ： When EXISTS When the conditional statement in can return the record line , Condition is true , Returns the current loop This record I got ; conversely , At present loop The record we got was discarded .
NOT EXISTS keyword ：NOT EXISTS And EXISTS contrary , That is to say EXISTS When the condition has a result set returned ,loop The records arrived will be discarded , Otherwise it would be loop The records to are added to the result set .

Count the students who did not take all the exams

mysql> SELECT *    -> FROM t_stu_profile b    -> WHERE NOT EXISTS(    ->     SELECT *    ->     FROM t_score a    ->     WHERE a.stu_id = b.stu_id)    -> ;+--------+----------+--------+------+----------+| Stu_id | Stu_Name | Gender | Age  | Class_id |+--------+----------+--------+------+----------+|      5 |  Wang Wu      | F      |   17 | 0614     ||      6 |  Zhao Qi      | F      |   16 | 0615     |+--------+----------+--------+------+----------+

2. function

①COUNT()

COUNT() Functions can be used in two ways ：
①COUNT(*)： Count the number of rows in the table , Whether a table column contains null values （NULL） Or non null ;
②COUNT(col_name)： Count rows with values in a specific column , Ignore NULL value ;
③COUNT(col_order)： The effect same as above , When col_order=1, Then it means that the 1 Column to count . Application scenarios ： Find the number of records of each course

# COUNT(*)mysql> SELECT lesson_id, COUNT(*) AS nums    -> FROM t_score    -> GROUP BY lesson_id    -> ;+-----------+------+| lesson_id | nums |+-----------+------+| L001      |    4 || L002      |    4 || L004      |    1 || L003      |    3 || L005      |    2 |+-----------+------+# COUNT(col_name)mysql> SELECT lesson_id, COUNT(lesson_id) AS nums    -> FROM t_score    -> GROUP BY lesson_id    -> ;# COUNT(col_order)mysql> SELECT lesson_id, COUNT(2) AS nums    -> FROM t_score    -> GROUP BY lesson_id    -> ;

②GROUP_CONCAT()

GROUP_CONCAT() function ： Realize grouping aggregation

GROUP_CONCAT(id ORDER BY id DESC SEPARATOR ‘_’)

③SUBSTRING_INDEX()

SUBSTRING_INDEX() function ： segmentation

SUBSTRING_INDEX(str, delim, count)
str： Intercepted field
delim： Separator
count： Count ,count Being positive , Left to right , Take the first place n All of the contents to the left of the separator ;count Negative , From right to left , Take the first place n All the contents to the right of the separator

Application scenarios ： Find the highest score of each subject
Implementation steps ：
① Will table t_score Press lesson_id grouping ,GROUP_CONCAT(score ORDER BY score DESC SEPARATOR '_');
② utilize SUBSTRING_INDEX(t.scores, '_', 1) Function .

mysql> SELECT t.lesson_id, SUBSTRING_INDEX(t.scores, '_', 1) AS max_score    -> FROM(    ->     SELECT lesson_id, GROUP_CONCAT(score ORDER BY score DESC SEPARATOR '_') AS scores    ->     FROM t_score    ->     GROUP BY lesson_id) t    -> GROUP BY t.lesson_id    -> ;+-----------+-----------+| lesson_id | max_score |+-----------+-----------+| L001      | 100       || L002      | 91        || L003      | 86        || L004      | 75        || L005      | 98        |+-----------+-----------+

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