Please check your join conditions. You may have missed adding one/more field(s) that are required for the join.
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It sounds like you have join duplication going on. This happens when your join key is not unique in one of the data sources. For example, if both data sources had 2 columns "first name" and "last name", you might only be joining on "first name". In this example, you would create a new row for every time someone had the same first name.
EX: Table 1
Joining on only first name and selecting all would yield:
John Doe | John Cena
John Rick | John Cena
John Doe | John Doe
John Rick | John DoeJames Verne | NULL
This is how duplicate rows are generally created. Here is a more technical description of this issue:
sql - Why do multiple-table joins produce duplicate rows? - Stack Overflow