SQL Joins

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  1. SQL Joins
    1. Tips
      1. ON vs WHERE

Tips

ON vs WHERE

The following queries are not interchangeable: ON clauses are evaluated at match time, while WHERE clauses are evaluated after matching. 

SELECT *
FROM transactions
LEFT JOIN people ON people.ID=transactions.ID
WHERE transactions.ID = 1

This will return only the rows in people where ID = 1. 

SELECT *
FROM transactions
LEFT JOIN people ON people.ID=transactions.ID
    AND transactions.ID = 12345

This will return all rows in people, but joined columns will only be non-NULL for the rows where ID = 1.

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