1) wht is the difference between concatenate and union?
A) Concat is an aggregation function.
SQL unions require that the same fields, with the same data types exist in each SELECT in the same order.
QV concatenates do not require any of these, but 'missing' fields will get null values.
2)wht is the difference between Fact and dimension?
Fact tables are having value fields like currency,Product No..etcand these tables are having lot of fields
Here data is modified very frequently (Ex:- Share Market data,Bank data)
Dim Tables are having very less data and these tables are explaining perticularly about only one thing(ex:-bank branches in all cities,type of accounts maintain in a bank..etc)
Here data is not modified frequently.
3)wht is the difference between concatenate and join?
For joining two tables one filed should be comman in between two tables.
concatenation: combining the two tables if no common fields
Concatenating fact tables with some different fields is acceptable for creating sophisticated Data scheme,
it is not out of best practice, just depends on complexity of your ap
Two different concatenation is there Auto Concatenation and Force Concatenation
When Qlikview sees the two different tables with same number of fields and same field names. It will concatenated in to single table by itself.
Using Concatenate keyword before the Load statement. For the non-common fields between two tables will have the NULL value in other table rows.
4)wht is the difference between star and snowflake schema?
If your database is normalized then snowflake is used, but there are performance issues using this schema, because at the number of joins are increased. That is why in Qlikview Star Schema is preferred, snowflake is converted to star by joining the Fact and Sub Fact tables. For less data you can't the performance issues, but when the data is huge you can notice the performace issues.
SnowFlake : Region (Table) -> Country(Table) -> City (Table) (All are separate fact tables)
Star : One table has all the fields (by joining all the tables)