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In some databases, such as Postgre SQL, when a FROM clause is present, what essentially happens is that the target table is joined to the tables mentioned in the fromlist, and each output row of the join represents an update operation for the target table.
When using FROM, one 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).
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.
Adding a WHERE clause would limit the result set of the JOINed table as well.
@Roger Ray what version of My SQL and what was your query, as this DOES infact function as stated. Col2)) UPDATE CTE SET Col1 = _Col1, Col2 = _Col2 statement on its own first to sanity check the results but it does requires you to alias the columns as above if they are named the same in source and target tables.
The CTE result set is derived from a simple query and is referenced by UPDATE statement.
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DEFAULT Specifies that the default value defined for the column is to replace the existing value in the column.
For example, you can use the MERGE statement to perform the following operations: It is important to understand how the source and target data are merged into a single input stream and how additional search criteria can be used to correctly filter out unneeded rows.
Otherwise,you might specify the additional search criteria in a way that produces incorrect results.