Multiple left join execution processes

select a.cName 
from table1 a 
LEFT JOIN table2 b ON a.code=b.code 
LEFT JOIN table3 c ON a.code=c.code The

process is as follows:
1. First, table1 left link to table2 to get an intermediate result, the The intermediate result includes all rows in table1 and row 2 in table2 that matches the condition (a.code=b.code) of table1
. The intermediate result is left-linked to table3 to obtain the final row set, which includes the intermediate results of the previous step. All rows and rows in table3 that match the condition (a.code=c.code) in the intermediate result

 

 

 ----- In general, it is to connect from left to right, and then join after left join, which will lead to the disappearance of the effect of left join, or all of them should be left join.

 

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