SQL problems encountered at work: Field a of table A is updated to field b of table B & field c of table A is refreshed based on field a.

1: Requirements introduction

1: The a field of table A is updated to
the value of the name field of the user table of the b field of the B table, and is synchronized to the user_name word of the user_info table.

2: The c field of table A is refreshed based on the a field. The
names field of the demo table stores multiple names, separated by English commas.

2: Update field a of table A to field b of table B

user table

user_info table

## 更新book表中的author字段 变成user表中的name
update  user_info ui set user_name = (
select u.name from user u where u.id = ui.user_id
)
// where ui.user_id in (select id from user )
## 这里 where 条件必须检查下id相同的才更新，否则不在user表的数据，user_name字段会被更新为null

Results of the

ps: This batch operation will be very slow for tables with large amounts of data.

Three: The c field of table A is refreshed based on the a field.

The count field of the demo table counts how many names there are in names

Query sql

select *,IF
	((
			LENGTH( names )- LENGTH(
			REPLACE ( names, ',', '' )) + 1 
			) IS NULL,
		0,(
			LENGTH( names )- LENGTH(
			REPLACE ( names, ',', '' )) + 1 
		)
	)  
	from demo

search result:

update sql

## 统计每行数据有多少个名字
update demo set count = IF
	((
			LENGTH( names )- LENGTH(
			REPLACE ( names, ',', '' )) + 1 
			) IS NULL,
		0,(
			LENGTH( names )- LENGTH(
			REPLACE ( names, ',', '' )) + 1 
		)
	) ;

Update result

ps: I only record the SQL problems encountered at work. I am Zha Zhatao, and I have to study hard.