One common reason can be due to mixed cases within the data of the join fields between the two datasets. For example, and as shown below, "Orange" from the first dataset is not going to join to "orange" in the second dataset. And this does not just apply to the leading character. Another common reason can be due to the existence of trailing or leading spaces. As also shown in the example below, "Plum " is not going to join with "Plum". The little red triangle in the upper right-hand corner of "Plum" indicates that there are leading and/or trailing spaces.
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