HQL Statement of Hive operation

HQL operation
1. Distribute by
Distribute by Col Scatter data across Col columns to different reduce
Sort
Sort by col sorts data according to Col column
Select COL,CO2 from table_name distribute by col1 Sort by col1
Asc,col2 desc;
The combination of both ensures that the output of each reduce is orderly

Application Scenarios:
* The file size of map output is not uniform
* Reduce output file size is not uniform
* Too many small files
* File size is very large
2. Cluster by
put the same worth of data together and sort, for example:
Cluster by Col
Distribute by col order by Col
3. Union All
merging data from multiple tables into a single table, hive does not support union
Select col from (select a as col from T1 union all select B as col from T2) TMP
Requirements:
field names are the same
Same as field type
The same number of fields
child tables cannot have aliases
If you need to query data from a merged table, the merged table must have an alias

HQL Statement of Hive operation