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Orders AS O GROUP BY O.customer_id )OA ON C.customer_id = OA.customer_id; table while pulling in necessary information from other table(s) referenced in the statement.If CTEs are your thing, you can even go a step further with this.If it does, then only one of the join rows will be used to update the target row, but which one will be used is not readily predictable.Because of this indeterminacy, referencing other tables only within sub-selects is safer, though often harder to read and slower than using a join.According to the standard, the source value for a parenthesized sub-list of column names can be any row-valued expression yielding the correct number of columns.
The sub-query can refer to old values of the current row of the table being updated. In fact, I struggled with this problem for quite some time before I found out about this gem.In the following lines, I am going to show you three tricks that have made my life simpler on many occasions.you should ensure that the join produces at most one output row for each row to be modified.In other words, a target row shouldn't join to more than one row from the other table(s).
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If you are not using the NUMBER(*) function, avoid using the ORDER BY clause, because the UPDATE statement performs better without it.