Most of what you’ll do with T-SQL centers around the Data Manipulation Language (DML). DML consists of the four statements SELECT, INSERT, UPDATE, and DELETE.
These are the four statements that you use to get data into a database,
update and delete that data, and get it back out again.
SELECT
The SELECT DML statement allows users to retrieve data from the database. A simplified version of the syntax follows:
SELECT <select_list> FROM <table_source> [WHERE <search_condition>]
where the parameters are defined as follows:
<select_list> specifies the columns that you want to return in the query.<table_source> is the table that you want to retrieve data from.<search_condition> is used to filter or scope the results that you are seeking.
The WHERE clause is optional. If you omit this clause, you will return all the rows from the given table. You can think of the WHERE clause like a Boolean expression. That is, the expression will be evaluated across every row in the table.
Basic SELECT Queries
Let’s look at an example. If you wanted to return all the pets’ names and weights, you would issue the following SELECT query:
SELECT pet_name, pet_weight FROM Pets
This statement would return the following result set:
pet_name pet_weight
Zeus 185
Lady 155
Deno 50
Rex 44
Rover 15
If you wanted to narrow the result set, you could add a WHERE clause like the following:
SELECT pet_name, pet_weight FROM Pets WHERE pet_weight > 100
This WHERE expression would cause the result set to return only pets whose weight was greater than the value 100.
In the example, you explicitly asked SQL Server to return specific columns from the table. You could have used an asterisk (*)
instead, and this would have returned all the columns in the table. To
return all the columns in the Pets table, you would issue the following
statement:
SELECT * FROM Pets
Sometimes tables have millions or billions of
rows, and you don’t really want to enumerate all of them at once. There
are expressions like TOP that you can use to limit the result set. TOP specifies that only a specific number of results or percentage of results will be returned from the query. For example, SELECT TOP 3 * FROM Pets will return the first three pets stored within the table, as shown here:
Pet_id pet_name pet_weight MicroChipID
1 Zeus 185 398BF49
2 Lady 155 191ABBC
3 Deno 50 790AG441
This limiting capability is also useful when used in conjunction with other clauses defined within the query. The ORDER BY
clause is used to instruct SQL Server to order the result set in a
particular way. You specify the columns to sort by and the direction of
the sort as parameters of the ORDER BY clause. For example, if you wanted to know the top three lightest pets, you would issue the following query:
SELECT TOP 3 * FROM Pets ORDER BY pet_weight ASC
This would yield the following result:
Pet_id pet_name pet_weight MicroChipID
5 Rover 15 A8719841
4 Rex 44 CDD81322
3 Deno 50 790AG441
The ASC parameter instructs SQL Server to order the results in ascending order, or lowest to highest. If you wanted to go the other way and order from highest to lowest, you could have specified DESC, which means descending.
A key value for relational databases is the
fact that sometimes different tables hold data that is related to each
other. This data can be combined and returned in a single result set.
In this example, you have a Pets table and an Owners
table. Pets have owners, so if you wanted to figure out which owners
have pets with a weight greater than 100, you could accomplish this by
joining the tables within the query. When you join together tables, you
have the option of specifying what to do with the rows that are matched
and those that are not matched. What happens depends on what kind of JOIN you include in your query. For example, if you wanted to know all the pets who have owners, you could execute this statement:
SELECT pet_name,owner_name FROM Pets
INNER JOIN Owners
ON Pets.pet_id=Owners.pet_id
This statement will yield the following results:
pet_name owner_name
Lady Bryan
Deno Rob
Zeus Rob
If you look at the data in both the Pets and Owners tables, you can see that two additional pets, Rex and Rover, were not displayed. This is because the Owners table does not have a matching pet_id for these two pets.
If you wanted to list all the pets regardless of whether they had an owner, you could issue a LEFT OUTER JOIN, which will always include the data from the table defined on the left side of the JOIN statement. The following is an example of listing all the pets and their owners, if they have any:
SELECT pet_name,owner_name FROM Pets
LEFT OUTER JOIN Owners
ON Pets.pet_id=Owners.pet_id
This statement will yield the following result:
pet_name owner_name
Zeus Rob
Lady Bryan
Deno Rob
Rex NULL
Rover NULL
Notice that NULL values result
when owners do not exist. Rex has no owner name on record. The system
doesn’t make up a name where none exists. When no value exists, the
system will return NULL.
Note INNER JOIN is the default join if you do not specify the join type.
There are other types of joins: RIGHT OUTER joins are similar to LEFT OUTER joins except they include the result set for the table to the right of the JOIN statement. A FULL OUTER join lists all results for both tables to the left and right of the JOIN statement.
Paging Support
When you want to view a product catalog on a
web page, you usually see the results returned from 1–25 for example.
The actual query, though, might have returned hundreds or thousands of
results. A lot of developers spent time writing complex code in the
middle-tier web page layer or the database layer to parse the full
query for the specified range. Starting in SQL Server 2012, native
paging support is available that allows you to ask for specific ranges
of rows to be returned. To illustrate, let’s add a few more rows of
data to our Pets table:
INSERT INTO Pets
VALUES(6,'Roscoe',55,'F5CAA29'),
(7,'Missy',67,'B7C2A59'),
(8,'George',12,'AA63BC5'),
(9,'Spot',34,'CC8A674')
GO
If you wanted to know the three heaviest pets, you could leverage the TOP T-SQL statement as follows:
SELECT TOP 3 pet_name,pet_weight FROM Pets ORDER BY pet_weight DESC
This would get the answer, but it would be difficult to find out the next three heaviest pets. This is where the OFFSET and FETCH statements work well. Consider the following statement, which uses OFFSET (0) to specify stating at the beginning and FETCH NEXT 3 ROWS to request only the three heaviest pets:
SELECT pet_name,pet_weight FROM Pets
ORDER BY pet_weight DESC
OFFSET (0) ROWS FETCH NEXT 3 ROWS ONLY
Running this statement yields the same result as our TOP 3 statement:
pet_name pet_weight
Zeus 185
Lady 155
Missy 67
If you wanted to know the next three, you would
set the offset to start at the third result and fetch the next three.
Notice the use of OFFSET (3) in the following example:
SELECT pet_name,pet_weight FROM Pets
ORDER BY pet_weight DESC
OFFSET (3) ROWS FETCH NEXT 3 ROWS ONLY;
This would yield the following result:
pet_name pet_weight
Roscoe 55
Deno 50
Rex 44
SELECT is arguably the most frequently used DML statement. Many different expressions and clauses are possible to use with SELECT.
Mastering the use of this statement will be important in your daily use
of SQL Server or any other database platform.
