SQL statement Considerations

1 Two Tables update the C field values in the T2 table to the C field in the T1 table based on the A field association
Update T1
Set T1. C =
(Select T2. C from T2 where T1. A = T2. A
where exists (select 1 from T2 where T1. A = T2. A);

The above SQL in the condition where exists (select 1 from T2 where T1. A = T2. A) must be written, otherwise all records in the T1 table will be updated, and the values of the records not associated to the record are updated for null values.

The results of the multi-table association are treated as a table and then associated with the source table.

SQL statement Considerations