The EXCEPT keyword in TSQL is used to find what is different between two datasets. The two datasets must have the same number of columns in them and use the same datatypes. The datasets must also have the columns in the same comparative order. The EXCEPT T-SQL Expression places the EXCEPT keyword between two datasets. By using the EXCEPT keyword between two datasets, TSQL will return a new dataset that just contains what is in the first dataset and not in the second dataset. The EXCEPT keyword is often used in this fashion:
SELECT col1,col2,coln... FROM table1 '[DataSet #1]
SELECT col1,col2,coln... FROM table2 '[DataSet #2]
The above image represents the use of EXCEPT between two datasets. The top circle (red) represents the first dataset, and the bottom circle (blue) represents the second dataset. The overlapping area is the section that is common to both datasets. By using the EXCEPT keyword between two datasets (red & blue circles), TSQL will return a new dataset that just contains the records that are unique to the first dataset (red) and not contained in the second dataset (blue), or the overlapping area.
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