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Learn SQL: Microsoft SQL Server - Episode 2: Selecting Records
Selecting a record or records from a table is the most common function that we will be performing in a database. We are going to discuss how to SELECT records from a table. We do this by following a specific syntax.
SELECT [Column Name]We choose a Column Name meaning the column in the table we want to select and pull the data from.
FROM [Table]After that we specify from which table we want to pull the data. This will pull the value from the specified column of all the rows in that table.
We can specify multiple columns as well using the following syntax.
SELECT [Column Name1], [Column Name2] , [Column Name3],...
FROM [Table Name]As you can see the Column Name is separated by a comma, and we can specify as many columns as we prefer.
We can use the following syntax when we want to pull data from all the columns.
SELECT *
FROM [Table]We can see how this works in SSMS.
In SSMS we need to select the New Query button. By default it will choose the master database. However we will be mostly working on the AdventureWorks database during our discussions.
We choose AdventureWorks at the dropdown menu, then we need to see which tables we have available in the AdventureWorks table. We navigate through the database structure Databases>AdventureWorks>Tables. In here we will see the tables available to us.
As we can see the tables have a schemaName which prefixes the actually table name. This lets us know who this table belongs to. When we query a table we need to specify the entire name, including the schemaName.
When you are the owner of a table, which is the case with any table that has the schemaName as db0, we do not need to specify the schemaName along with the table name.
Although we can refer to it with the schemaName as this works as well.
Let's start selecting and pulling data from the Person.Person table.
This table has the information of every single person whether it is a customer or an employee. We need to click on plus sign on the left of the Person.Person table to see which columns are available. Person.Person>Columns. Inside the Columns structure we will see all the columns.
Lets say for instance we want to pull the first name of every single person in this table. We type our query using the correct syntax and when complete, we hit the Execute button.
Select firstname
From Person.Person
Notice how intuitive SSMS is, it can detect which tables and columns are available to choose from and present them to us for quick access.
All the statement we write in SSMS are case insensitive.
Let’s select multiple columns from the Person.Person table. We want to pull the firstName, middleName and lastName from the Person.Person table. To do this we separate the columns by a comma.
Also the spacing between commas do not matter. But for readability keep everything consistent.
Select firstname, middleName, lastName
From Person.Person
SSMS not only shows us the columns available in an intuitive way but it also shows us any functions that are available to us in SSMS.
Now let’s select all the columns in the Person.Person table.
If you want to keep your current query statement, then we can leave it as is. We can hit the New Query button and a new window will open, where we can write our new query statement. Also our previous statement will still be accessible by jumping between windows.
We can also write the new query statement below our current query statement, if we hit Execute after writing the new query we will get the results from both queries. But if we only highlight our new query and hit Execute, only that query statement will be executed, the same applies to the query above.
We can also save our queries, which allows us to use them at a later stage. We have to hit the SAVE icon, then we can name it and click save. Notice that only the query statements of the active window was saved, not the other window.
We can use the saved query by navigating at the top-left to file>open>file or by using the ctrl + o key combination. Select our saved query and hit Open. Notice that only the query was saved and not the result set.
When we have a large database, there are millions of records in our table. Then it's not feasible to select all the records from the table, if all we want to do is to just have a simple set.
We are going to discuss the queries we can use to select only a specified number of records from a table.
There are two ways to specify the number of records we want to pull from a table.
We can specify the number of records we want to pull, for example 150 or 800 records. We can also specify the percentage of records we want to pull, for example 30% or 5%.
We are going to experiment with how you can limit the number of records we can pull from a table by using both types of criteria.
We use the following syntax to query a table for the TOP N number of records:
Select top N [Column Names or *]
From [Table Name]N is the number of records we want to pull and Column Names are the columns we want to select, we can choose as many columns as we prefer or specify the asterisk * symbol to select all columns.
After that we specify the table we want to pull the records from.
We use the following syntax to query a table for the TOP N Percentage of records:
Select top N Percent [Column Names or *]
From [Table Name]N is the number and then Percent of records we want to pull. Column Names are the columns we want to select, we can choose as many columns as we prefer or specify the asterisk * symbol to select all columns.
For example:
Select top 5 Percent [Column Names or *]
From [Table Name]
After that we specify the table we want to pull the records from.
This is the way we can limit the amount of data we get from the database and make our queries go faster, most of the time for analysis, this might be all we require.
It could be we only want to see the type of data in a table, then we will most likely use the TOP 2 or TOP 5. It could be we need to do some analysis, manipulation, experimentation on a few records, then we most likely will use TOP 2 Percent or TOP 5 Percent to conduct our analysis.
It is better to get familiar with these commands since they are the most basic and frequently used. Practice and get better acquainted with them. Try them on other tables and use different columns 😉
Play around with the AdventureWorks database and soon we will be diving deeper into the world of SSMS.
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