MySQL implementation of field segmentation from one line to multiple lines of example code

Let's look at the data structure first , I have few fields here , Only the most important part

According to what we learned last time LEFT() Function to group

SELECT LEFT(provinces,6),COUNT(1) FROM `region_map_copy` GROUP BY LEFT(provinces,6)

The result is as follows ：

This effect is not what we want , We want to 210000 All are merged

Use SUBSTRING_INDEX(),LEFT() These two functions , In Canada help_topic This table

help_topic： Split by character , One line to many lines

SELECT COUNT(*), SUBSTRING_INDEX( SUBSTRING_INDEX( a.provinces, ',', b.help_topic_id + 1 ), ',',- 1 ) AS ids FROM `region_map_copy` AS a JOIN mysql.help_topic AS b ON b.help_topic_id < ( LENGTH( a.provinces ) - LENGTH( REPLACE ( a.provinces, ',', '' ) ) + 1 ) GROUP BY (SUBSTRING_INDEX( SUBSTRING_INDEX( a.provinces, ',', b.help_topic_id + 1 ), ',',- 1 ) )

give the result as follows ：

Got what we wanted

mysql Field interception function ：

left( Field name ,index) Start from the left index Began to intercept

right( Field name ,index) Start from the right index Began to intercept

substring( Field name ,index) When index>0 From the left to the end When index<0 From the right to the end When index=0 Returns an empty

substring( Field name ,index,len) from index Start , Intercept len length

substring_index( Field name ,str,count),str Is the intercepted field count Where does it start (0 From the left, the second 0 Start ,-1 Start first on the right ）

Be careful ： This way does not support mariadb

Add ： Look at the below MySql The fields in the database table are separated by commas , Field for conditional query

select p.* from t_project_info p

select p.*from t_project_info pwhere  FIND_IN_SET(6,p.thematic_library_ids)

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