Use IN clause with care

Posted on by madhivanan

IN clause can be used as a JOIN to get data by matching the column values from other table. Although this is much similar to INNER JOIN, be careful about the usage of IN

Let us create the following data sets

use tempdb 

create table customer_master
(
cust_id int,
cust_name varchar(100)
);

insert into customer_master(cust_id,cust_name)
select 1,'Sankar' union all
select 2,'Madhivanan' union all
select 3,'Gopal';

create table premium_customers
(
cust_id int
);


insert into premium_customers(cust_id)
select 1 union all
select 2

Now to get all customers who are in premium_customers table, you can use this

select 
	* 
from 
	customer_master 
where 
	cust_id in 
		(
		select cust_id from premium_customers
		)

which returns the following result

cust_id     cust_name
----------- -----------
1           Sankar
2           Madhivanan

Now, let us re name cust_id of premium_customers into customer_id

exec sp_rename 'premium_customers.cust_id','customer_id','COLUMN'

Now execute the same query

select 
	* 
from 
	customer_master 
where 
	cust_id in 
		(
		select cust_id from premium_customers
		)

The result is

cust_id     cust_name
----------- -------------
1           Sankar
2           Madhivanan
3           Gopal

It returns ALL ROWS from customer_master table although there is no column named cust_id in premium_customers. Because it is a column in customer_master table, it is matched with customer_master table and gives all rows

How do we avoid this?

Always use table alias and qualify the columns using table alias

Executing the following code

select 
	t1.* 
from 
	customer_master as t1
where 
	t1.cust_id in 
		(
		select t2.cust_id from premium_customers as t2
		)

now throws an error

Msg 207, Level 16, State 1, Line 39
Invalid column name 'cust_id'.

So it is always a better practice to use table alias and qualify the columns with it.

Avoid view creation with “SELECT *”

Posted on by madhivanan

A view is very useful in giving out only the required/restricted information to the users. However see what happens when you create a view with “SELECT * from table” definition.

Let us create this simple table

create table emp_master
(
emp_id int,
emp_name varchar(100),
joined_date date
)

Create a view based on this table using “SELECT *”

Create view emp_master_view as
select * from emp_master;

Now see the column information of this view using sp_columns system procedure

EXEC sp_columns 'emp_master_view'

The result is shown in the below imageview1

Now let us add a new column in the emp_master table

alter table emp_master add address varchar(200)

Now see the column information of view using sp_columns system procedure

EXEC sp_columns 'emp_master_view'

The result is shown in the below image. As you see the address column is not yet updated in the view.view1

So what is the proper way to update the column information of the view?

Use sp_refreshview system rocedure

EXEC sp_refreshview emp_master_view

After executing the sp_refreshview procedure, let us see the column information

EXEC sp_columns 'emp_master_view'

As you see in the following result, new column is added in the viewview2

Points to consider when creating a view

1 Do not use *. Explicitly type out the column names in the view definition. When new columns are required, alter the view to add them
2 Use schemabinding option when creating view which will prevent the usuage of *