I read some articles but really didn't understand what does select 1 from do? Someone says "you should use select 1 instead of select *". Here is an example table:
cust_id cust_name cust_address
1000000001 Village Toys Mapl
1000000002 Kids Place South
1000000003 Fun4All Sunny
1000000004 Fun4All Riverside
1000000005 The Toy Store 53rd
What will the result be when I write select 1 from customer_table what does this statement do?
5 Answers
select 1 from table
will return a column of 1's for every row in the table. You could use it with a where statement to check whether you have an entry for a given key, as in:
if exists(select 1 from table where some_column = 'some_value')
What your friend was probably saying is instead of making bulk selects with select * from table, you should specify the columns that you need precisely, for two reasons:
1) performance & you might retrieve more data than you actually need.
2) the query's user may rely on the order of columns. If your table gets updated, the client will receive columns in a different order than expected.
The construction is usually used in "existence" checks
if exists(select 1 from customer_table where customer = 'xxx')
or
if exists(select * from customer_table where customer = 'xxx')
Both constructions are equivalent. In the past people said the select * was better because the query governor would then use the best indexed column. This has been proven not true.
It does what you ask, SELECT 1 FROM table will SELECT (return) a 1 for every row in that table, if there were 3 rows in the table you would get
1
1
1
Take a look at Count(*) vs Count(1) which may be the issue you were described.
The statement SELECT 1 FROM SomeTable just returns a column containing the value 1 for each row in your table. If you add another column in, e.g. SELECT 1, cust_name FROM SomeTable then it makes it a little clearer:
cust_name
----------- ---------------
1 Village Toys
1 Kids Place
1 Fun4All
1 Fun4All
1 The Toy Store
SELECT COUNT(*) in EXISTS/NOT EXISTS
EXISTS(SELECT CCOUNT(*) FROM TABLE_NAME WHERE CONDITIONS) - the EXISTS condition will always return true irrespective of CONDITIONS are met or not.
NOT EXISTS(SELECT CCOUNT(*) FROM TABLE_NAME WHERE CONDITIONS) - the NOT EXISTS condition will always return false irrespective of CONDITIONS are met or not.
SELECT COUNT 1 in EXISTS/NOT EXISTS
EXISTS(SELECT CCOUNT 1 FROM TABLE_NAME WHERE CONDITIONS) - the EXISTS condition will return true if CONDITIONS are met. Else false.
NOT EXISTS(SELECT CCOUNT 1 FROM TABLE_NAME WHERE CONDITIONS) - the NOT EXISTS condition will return false if CONDITIONS are met. Else true.
