Summary of three methods for SQL deduplication

A summary of three methods of SQL deduplication

Deduplication here means: when querying, do not display duplicates, not delete duplicates in the table

1.distinctDe-duplication
Points to note: distinct
can only de-duplicate in one column. When distinct is followed by more than 1 parameter, the relationship between them is &&(logicand) relationship, only all conditions are the same to deduplicate
Disadvantage: When there are many fields in the query, distinct will act on multiple fields, resulting in more deduplication conditions
select distinct UserResult from Table1

2.group byDe-duplication
Principle of de-duplication: group duplicate rows, and only display the first row of the same data
Disadvantage: After using group by, all queriesAll fields need to use aggregate functions, which is cumbersome
select min(UserName)UserName,min(UserSex)UserSex,min(UserSubject)UserSubject,min(UserResult)UserResult from Table1
group by UserResult

3.row_number() over (parttion by grouping column order by sorting column)
Disadvantage: Xiaomeng doesn't know yet
Deduplication principle: Now group by repeating column, and then sort after grouping, differentThe group number is 1, the same group number is 2, and the deduplication effect is achieved if the group number is 2.
select *from
(
– query duplicate rows
select *,row_number()over (partition by UserResult order by UserResult desc)num from Table1
)A
where A.num=1

Here AmwayThe third one, row_number(), be steady!

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