What's the difference between NOT EXISTS vs. NOT IN vs. LEFT JOIN WHERE IS NULL?

Question

It seems to me that you can do the same thing in a SQL query using either NOT EXISTS, NOT IN, or LEFT JOIN WHERE IS NULL. For example:

SELECT a FROM table1          


        

Answer 1

Assuming you are avoiding nulls, they are all ways of writing an anti-join using Standard SQL.

An obvious omission is the equivalent using EXCEPT:

SELECT a FROM table1
EXCEPT
SELECT a FROM table2

Note in Oracle you need to use the MINUS operator (arguably a better name):

SELECT a FROM table1
MINUS
SELECT a FROM table2

Speaking of proprietary syntax, there may also be non-Standard equivalents worth investigating depending on the product you are using e.g. OUTER APPLY in SQL Server (something like):

SELECT t1.a
  FROM table1 t1
       OUTER APPLY 
       (
        SELECT t2.a
          FROM table2 t2
         WHERE t2.a = t1.a
       ) AS dt1
 WHERE dt1.a IS NULL;