Saturday, 21 August 2021

Join Order --- Left Join with Inner Join - Control Join Precedence Order

In SQL all join conditions have the same precedence so 

like in mat 1 -2 + 3   = (1 - 2) +3 and not 1 - (2 + 3)

similarly in SQL T1 Left Join T2 Inner Join T3 is evaluated as (T1 Left Join T2) Inner Join T3  

If the T3 Join condition doesn't return anything, it will make the output of  T1 and T2 also not show up in output. 


 Select * from  T1

Left Join (T2 left join T3 on T2.id = T3.ID ) 

on T2.id =T1.id

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