Detailed explanation of mysql exists and not exists examples

Detailed explanation of mysql exists and not exists examples

tableA

|column1 | column1 | column3 |

tableb

|column1 | column1 | column3 |

To query the data of tableA, the condition is that tableA.column1 is not in tableB.column2 of tableB

That is to say, you want to get an effect similar to the following statement (the effect of not in is not completely equivalent to not exists. If there is an empty record in the subquery, the entire query statement will not return data)

SELECT 
  a.*
FROM
  table A
WHERE 
  a.column1 not in (
    SELECT column2 FROM tableB
  )

This can be achieved using the following statement

SELECT
  a.*
FROM
  table A
WHERE
  NOT EXISTS(
    SELECT b.column2 FROM tableB b WHERE a.column1=b.column2
  )

The above is only the case of two tables. In fact, it is also easy to use in the connection query of multiple tables. The above writing method also applies to exists

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