This is one of the most interesting questions I keep on getting on this email and I find that not everyone knows about it. In recent times I have seen a developer writing a cursor to update a table. When asked the reason was he had no idea how to use multiple tables with the help of the JOIN clause in the UPDATE statement.
Let us see the following example. We have two tables Table 1 and Table 2.
-- Create table1
CREATE TABLE Table1 (Col1 INT, Col2 INT, Col3 VARCHAR(100))
INSERT INTO Table1 (Col1, Col2, Col3)
SELECT 1, 11, 'First'
SELECT 11, 12, 'Second'
SELECT 21, 13, 'Third'
SELECT 31, 14, 'Fourth'
-- Create table2
CREATE TABLE Table2 (Col1 INT, Col2 INT, Col3 VARCHAR(100))
INSERT INTO Table2 (Col1, Col2, Col3)
SELECT 1, 21, 'Two-One'
SELECT 11, 22, 'Two-Two'
SELECT 21, 23, 'Two-Three'
SELECT 31, 24, 'Two-Four'
Now let us check the content in the table.
Now let us see the following image. Our requirement is that we have Table2 which has two rows where Col1 is 21 and 31. We want to update the value from Table2 to Table1 for the rows where Col1 is 21 and 31. Additionally, we want to update the values of Col2 and Col3 only.
When you look at this it looks very simple but when we try to think the solution, I have seen developers coming up with many different solutions for example sometime they write cursor, table variables, local variables etc. However, the easiest and the most clean way is to use JOIN clause in the UPDATE statement and use multiple tables in the UPDATE statement and do the task.
SET Col2 = t2.Col2,
Col3 = t2.Col3
FROM Table1 t1
INNER JOIN Table2 t2 ON t1.Col1 = t2.Col1
WHERE t1.Col1 IN (21, 31)
Now let us select the data from these tables.
-- Check the content of the table
As you can see that using JOIN clause in UPDATE statement it makes it very easy to update data in one table from another table. You can additionally use MERGE statement to do the same as well, however I personally prefer this method. Let us clean up the clause by dropping the tables which we have created.
DROP TABLE Table1
DROP TABLE Table2
Do let me know if you use any other trick in similar situations. If you do, I would like to learn more about it.
Reference: Pinal Dave (http://blog.sqlauthority.com)