works fine when I try to update all the records in tbl A, however, in this case I only have missing data which I have identified and populated in tbl B.
When I try to update only the missing data the non-matched data is also updated but to NULL. The reason some of your rows are getting NULLs updated is due to the subquery.
If the table you're updating appears more than once in the FROM clause, only one reference to the table can omit a table alias; all other references to the table must include a table alias.
How can I update three columns in table A from three columns in table B in one UPDATE statement?
For this question, you can use the power of relational algebra.
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The process of updating data from one table to another follows these broad steps: The steps in this section assume the use of two similar tables.
In this example, the Clients table is located in a database that you just inherited, and it contains more recent data than the Customers table.
When the subquery fails to find a matching row in tbl B, the subquery returns NULL.
But since the UPDATE has no WHERE clause, it will update all rows, even those where the subquery returns NULL.
Consequently, to prevent this, you might do this: One potential issue may arise when the subquery actually finds more than one matching row in tbl B.