null in sql

A Null value in a record represents an empty value in a database table. It differs from a value that is zero as this is an integer, and therefore not an empty value.

SQL 

how to insert data into a table in SQL

admin 4340 Views 0 Comments , , , ,

Estimated reading time: 4 minutes

After building your data using a create statement, one of the next steps using SQL is to now get your data in, so it can be managed and updated where necessary.

There are many ways to import your data including , one you could use is import a CSV file with an SQL query.

Here we talk about some of the steps involved, and also how an understanding of the primary key is important.

Before all the following updates happen, the table looks like this:

--Inserting one row of data
Insert INTO dbo.CUSTOMER(CUSTOMER_NO,customer_type)
VALUES (456789,'web')
select * from dbo.CUSTOMER

Insert one row into a database table

In the below code we will use the INSERT statement that basically tells the SQL to add a new row to the table.

It is important to remember two things about the primary key:

  • It has to be unique.
  • There can be no null values.

As a result of this been enforced, it will allow tables to be joined together assuming they have the same primary key information.

The output gives us the following:

Inserting multiple rows of data into one table

Following on from the above , we may have a need to insert more than one row.

Again it would follow the same steps, except, we need to ensure all data been inserted follows the same structure and there are no duplicate values in the CUSTOMER_NO column.

Any duplicates in this column would cause it to fail as follows:

It would fail on 125872 and then 456789, both need to be removed from the insert. A sample error message is shown below.

Insert INTO dbo.CUSTOMER(CUSTOMER_NO,customer_type)
VALUES 
 (125872,'web'),
 (333333,'web'),
 (987981,'web'),
 (879784,'web'),
 (654654,'web'),
 (456789, 'Phone')
 select * from dbo.CUSTOMER

Removing the duplicate values from the insert statement, also allows the update to complete successfully:

Insert INTO dbo.CUSTOMER(CUSTOMER_NO,customer_type)
VALUES 
 (333333,'web'),
 (987981,'web'),
 (879784,'web'),
 (654654,'web')
 select * from dbo.CUSTOMER

Insert values into a particular column

There is going to be a scenario, from time to time where you only want to insert values into a particular column, but not all of them.

With this table, as it has a primary key, you can insert new values into CUSTOMER_NO, with no values required for customer_type, as shown below.

If there was no primary key, then you could use the below statement, and in the where clause identify those specific records that need updating.

--Insert values into a particular column
Insert INTO dbo.CUSTOMER(CUSTOMER_NO)
VALUES 
(25897458),
(25898633)
select * from dbo.CUSTOMER

This will yield in the output:

The next section will show you the error , when trying to insert into the customer_type, and leaving CUSTOMER_NO blank.

Inserting a value in without updating the primary key, will fail

Finally we will look at how trying to update a table with a primary key, without providing the primary key value will give you an error.

The code we will use is:

--Inserting a value in without updating the primary key, will fail
Insert INTO dbo.CUSTOMER(customer_type)
VALUES 
('phone'),
('web')
select * from dbo.CUSTOMER

But the table schema states that the CUSTOMER_NO cannot be null:

As a result the following error appears:

In summary we have taken you through a number of different steps , and some of the caveats that you need to understand before inserting data.

The most important thing is that if you have a primary key on a table, doing inserts needs a value supplied that is not a duplicate and also that no empty values for that column are supplied.

SQL SQLite 

How To Join Tables In SQL

admin 3373 Views 0 Comments , , , , , , ,

Estimated reading time: 4 minutes

When running an sql select query, most likely we may have a need to bring in data from other tables.

The reason for this is that tables have a specific purpose for the data they store.

One of the most important features of a data architecture is to avoid duplication.

This has the impact of only storing data on a table that is required.

As a result, joining tables is necessary if you want to get all the information you need .

As an illustration of customer purchase, the purchase details would be on the purchase table.

BUT the customer details would not be stored there. They would be stored on the customer table.

To put it another way, a customer may make many purchases, but you only need their name and address etc. stored once.

The object therefore of joining tables is to use this structure, and avoid duplication over many tables .

For example in the above table, if you had the customer name and address on the purchases table and customer table,

they would always have to be in sync, and it would bring up a maintenance headache for the database administrator.

So what was developed was primary and foreign keys, which help to join tables, based on unique similar values in a column in each table.

What are the types of joins that can be used?

Inner Join: An inner join is where you join two tables but only return the rows that match in both tables.

Left Join: A left join is where you return all the records from the left table, and any matched records from the right table.

Right Join: A right join is where you return all the records from the right table, and any matched records from the left table.

Full Join: A full join uses the left join and right join, and returns all the rows from both tables in the output.

Equally important, where there is no value found a NULL value will be returned.

Lets take some examples to explain the above concepts.

So we have two tables below:

Sales Database, Customer Table

Sales Database, Sales Table

Inner Join

If we run this code, we will return the below. For this reason, the purpose of an Inner Join is to only return matched records common to both.

select a.customer_no, a.customer_type, b.INVOICE_NO 
from dbo.CUSTOMER a -- left table
inner join sales.dbo.sales b -- right table
on a.CUSTOMER_NO = b.CUSTOMER_NO

Left Join

With a left join of the below code will return four rows. All the records from the left table and any matched records from the right table.

select a.customer_no, a.customer_type, b.INVOICE_NO 
from dbo.CUSTOMER a --left table
left join sales.dbo.sales b --right table
on a.CUSTOMER_NO = b.CUSTOMER_NO

Right Join

With a right join of the below code will return three rows. All the records from the right table and any matched records from the left table.

select a.customer_no, a.customer_type, b.INVOICE_NO 
from dbo.CUSTOMER a --left table
right join sales.dbo.sales b -- right table
on a.CUSTOMER_NO = b.CUSTOMER_NO

Full Join

Finally, in order to run a full join, the result of the below code would be as per the below. The objective is to return all rows matching and unmatching.

In the output, there is a Null Value, meaning that that row has no value for INVOICE_NO which is a primary key.

In essence, primary keys are not allowed to have null values.

select a.customer_no, a.customer_type, b.INVOICE_NO from dbo.sales b
full join sales.dbo.customer a
on a.CUSTOMER_NO = b.CUSTOMER_NO