The TOP Clause
The TOP clause is used to specify the number of records to return.
The TOP clause can be very useful on large tables with thousands of records. Returning a large number of records can impact on performance.
Note: Not all database systems support the TOP clause.
SQL Server Syntax
| SELECT TOP number|percent column_name(s) FROM table_name |
SQL SELECT TOP Equivalent in MySQL and Oracle
MySQL Syntax
| SELECT column_name(s) FROM table_name LIMIT number |
Example
| SELECT * FROM Persons LIMIT 5 |
Oracle Syntax
| SELECT column_name(s) FROM table_name WHERE ROWNUM <= number |
Example
| SELECT * FROM Persons WHERE ROWNUM <=5 |
SQL TOP Example
The "Persons" table:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
| 4 | Nilsen | Tom | Vingvn 23 | Stavanger |
Now we want to select only the two first records in the table above.
We use the following SELECT statement:
| SELECT TOP 2 * FROM Persons |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
SQL TOP PERCENT Example
The "Persons" table:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
| 4 | Nilsen | Tom | Vingvn 23 | Stavanger |
Now we want to select only 50% of the records in the table above.
We use the following SELECT statement:
| SELECT TOP 50 PERCENT * FROM Persons |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
The LIKE operator is used in a WHERE clause to search for a specified pattern in a column.
The LIKE Operator
The LIKE operator is used to search for a specified pattern in a column.
SQL LIKE Syntax
| SELECT column_name(s) FROM table_name WHERE column_name LIKE pattern |
LIKE Operator Example
The "Persons" table:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
Now we want to select the persons living in a city that starts with "s" from the table above.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE City LIKE 's%' |
The "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
Next, we want to select the persons living in a city that ends with an "s" from the "Persons" table.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE City LIKE '%s' |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
Next, we want to select the persons living in a city that contains the pattern "tav" from the "Persons" table.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE City LIKE '%tav%' |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
It is also possible to select the persons living in a city that NOT contains the pattern "tav" from the "Persons" table, by using the NOT keyword.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE City NOT LIKE '%tav%' |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
SQL wildcards can be used when searching for data in a database.
SQL Wildcards
SQL wildcards can substitute for one or more characters when searching for data in a database.
SQL wildcards must be used with the SQL LIKE operator.
With SQL, the following wildcards can be used:
| Wildcard | Description |
| % | A substitute for zero or more characters |
| _ | A substitute for exactly one character |
| [charlist] | Any single character in charlist |
| [^charlist] or [!charlist] | Any single character not in charlist |
SQL Wildcard Examples
We have the following "Persons" table:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
Using the % Wildcard
Now we want to select the persons living in a city that starts with "sa" from the "Persons" table.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE City LIKE 'sa%' |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
Next, we want to select the persons living in a city that contains the pattern "nes" from the "Persons" table.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE City LIKE '%nes%' |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
Using the _ Wildcard
Now we want to select the persons with a first name that starts with any character, followed by "la" from the "Persons" table.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE FirstName LIKE '_la' |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
Next, we want to select the persons with a last name that starts with "S", followed by any character, followed by "end", followed by any character, followed by "on" from the "Persons" table.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE LastName LIKE 'S_end_on' |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
Using the [charlist] Wildcard
Now we want to select the persons with a last name that starts with "b" or "s" or "p" from the "Persons" table.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE LastName LIKE '[bsp]%' |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
Next, we want to select the persons with a last name that do not start with "b" or "s" or "p" from the "Persons" table.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE LastName LIKE '[!bsp]%' |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
The IN Operator
The IN operator allows you to specify multiple values in a WHERE clause.
SQL IN Syntax
| SELECT column_name(s) FROM table_name WHERE column_name IN (value1,value2,...) |
IN Operator Example
The "Persons" table:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
Now we want to select the persons with a last name equal to "Hansen" or "Pettersen" from the table above.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE LastName IN ('Hansen','Pettersen') |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
The BETWEEN operator is used in a WHERE clause to select a range of data between two values.
The BETWEEN Operator
The BETWEEN operator selects a range of data between two values. The values can be numbers, text, or dates.
SQL BETWEEN Syntax
| SELECT column_name(s) FROM table_name WHERE column_name BETWEEN value1 AND value2 |
BETWEEN Operator Example
The "Persons" table:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
Now we want to select the persons with a last name alphabetically between "Hansen" and "Pettersen" from the table above.
We use the following SELECT statement:
| SELECT * FROM Persons WHERE LastName BETWEEN 'Hansen' AND 'Pettersen' |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
Note: The BETWEEN operator is treated differently in different databases!
In some databases, persons with the LastName of "Hansen" or "Pettersen" will not be listed, because the BETWEEN operator only selects fields that are between and excluding the test values.
In other databases, persons with the LastName of "Hansen" or "Pettersen" will be listed, because the BETWEEN operator selects fields that are between and including the test values.
And in other databases, persons with the LastName of "Hansen" will be listed, but "Pettersen" will not be listed (like the example above), because the BETWEEN operator selects fields between the test values, including the first test value and excluding the last test value.
Therefore: Check how your database treats the BETWEEN operator.
Example 2
To display the persons outside the range in the previous example, use NOT BETWEEN:
| SELECT * FROM Persons WHERE LastName NOT BETWEEN 'Hansen' AND 'Pettersen' |
The result-set will look like this:
| P_Id | LastName | FirstName | Address | City |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
With SQL, an alias name can be given to a table or to a column.
SQL Alias
You can give a table or a column another name by using an alias. This can be a good thing to do if you have very long or complex table names or column names.
An alias name could be anything, but usually it is short.
SQL Alias Syntax for Tables
| SELECT column_name(s) FROM table_name AS alias_name |
SQL Alias Syntax for Columns
| SELECT column_name AS alias_name FROM table_name |
Alias Example
Assume we have a table called "Persons" and another table called "Product_Orders". We will give the table aliases of "p" and "po" respectively.
Now we want to list all the orders that "Ola Hansen" is responsible for.
We use the following SELECT statement:
| SELECT po.OrderID, p.LastName, p.FirstName FROM Persons AS p, Product_Orders AS po WHERE p.LastName='Hansen' AND p.FirstName='Ola' |
The same SELECT statement without aliases:
| SELECT Product_Orders.OrderID, Persons.LastName, Persons.FirstName FROM Persons, Product_Orders WHERE Persons.LastName='Hansen' AND Persons.FirstName='Ola' |
As you'll see from the two SELECT statements above; aliases can make queries easier to both write and to read.
SQL joins are used to query data from two or more tables, based on a relationship between certain columns in these tables.
SQL JOIN
The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables.
Tables in a database are often related to each other with keys.
A primary key is a column (or a combination of columns) with a unique value for each row. Each primary key value must be unique within the table. The purpose is to bind data together, across tables, without repeating all of the data in every table.
Look at the "Persons" table:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
Note that the "P_Id" column is the primary key in the "Persons" table. This means that no two rows can have the same P_Id. The P_Id distinguishes two persons even if they have the same name.
Next, we have the "Orders" table:
| O_Id | OrderNo | P_Id |
| 1 | 77895 | 3 |
| 2 | 44678 | 3 |
| 3 | 22456 | 1 |
| 4 | 24562 | 1 |
| 5 | 34764 | 15 |
Note that the "O_Id" column is the primary key in the "Orders" table and that the "P_Id" column refers to the persons in the "Persons" table without using their names.
Notice that the relationship between the two tables above is the "P_Id" column.
Different SQL JOINs
Before we continue with examples, we will list the types of JOIN you can use, and the differences between them.
- JOIN: Return rows when there is at least one match in both tables
- LEFT JOIN: Return all rows from the left table, even if there are no matches in the right table
- RIGHT JOIN: Return all rows from the right table, even if there are no matches in the left table
- FULL JOIN: Return rows when there is a match in one of the tables
SQL INNER JOIN Keyword
The INNER JOIN keyword return rows when there is at least one match in both tables.
SQL INNER JOIN Syntax
| SELECT column_name(s) FROM table_name1 INNER JOIN table_name2 ON table_name1.column_name=table_name2.column_name |
PS: INNER JOIN is the same as JOIN.
SQL INNER JOIN Example
The "Persons" table:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
The "Orders" table:
| O_Id | OrderNo | P_Id |
| 1 | 77895 | 3 |
| 2 | 44678 | 3 |
| 3 | 22456 | 1 |
| 4 | 24562 | 1 |
| 5 | 34764 | 15 |
Now we want to list all the persons with any orders.
We use the following SELECT statement:
| SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo FROM Persons INNER JOIN Orders ON Persons.P_Id=Orders.P_Id ORDER BY Persons.LastName |
The result-set will look like this:
| LastName | FirstName | OrderNo |
| Hansen | Ola | 22456 |
| Hansen | Ola | 24562 |
| Pettersen | Kari | 77895 |
| Pettersen | Kari | 44678 |
The INNER JOIN keyword return rows when there is at least one match in both tables. If there are rows in "Persons" that do not have matches in "Orders", those rows will NOT be listed.
SQL LEFT JOIN Keyword
The LEFT JOIN keyword returns all rows from the left table (table_name1), even if there are no matches in the right table (table_name2).
SQL LEFT JOIN Syntax
| SELECT column_name(s) FROM table_name1 LEFT JOIN table_name2 ON table_name1.column_name=table_name2.column_name |
PS: In some databases LEFT JOIN is called LEFT OUTER JOIN.
SQL LEFT JOIN Example
The "Persons" table:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
The "Orders" table:
| O_Id | OrderNo | P_Id |
| 1 | 77895 | 3 |
| 2 | 44678 | 3 |
| 3 | 22456 | 1 |
| 4 | 24562 | 1 |
| 5 | 34764 | 15 |
Now we want to list all the persons and their orders - if any, from the tables above.
We use the following SELECT statement:
| SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo FROM Persons LEFT JOIN Orders ON Persons.P_Id=Orders.P_Id ORDER BY Persons.LastName |
The result-set will look like this:
| LastName | FirstName | OrderNo |
| Hansen | Ola | 22456 |
| Hansen | Ola | 24562 |
| Pettersen | Kari | 77895 |
| Pettersen | Kari | 44678 |
| Svendson | Tove | |
The LEFT JOIN keyword returns all the rows from the left table (Persons), even if there are no matches in the right table (Orders).
SQL RIGHT JOIN Keyword
The RIGHT JOIN keyword returns all the rows from the right table (table_name2), even if there are no matches in the left table (table_name1).
SQL RIGHT JOIN Syntax
| SELECT column_name(s) FROM table_name1 RIGHT JOIN table_name2 ON table_name1.column_name=table_name2.column_name |
PS: In some databases RIGHT JOIN is called RIGHT OUTER JOIN.
SQL RIGHT JOIN Example
The "Persons" table:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
The "Orders" table:
| O_Id | OrderNo | P_Id |
| 1 | 77895 | 3 |
| 2 | 44678 | 3 |
| 3 | 22456 | 1 |
| 4 | 24562 | 1 |
| 5 | 34764 | 15 |
Now we want to list all the orders with containing persons - if any, from the tables above.
We use the following SELECT statement:
| SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo FROM Persons RIGHT JOIN Orders ON Persons.P_Id=Orders.P_Id ORDER BY Persons.LastName |
The result-set will look like this:
| LastName | FirstName | OrderNo |
| Hansen | Ola | 22456 |
| Hansen | Ola | 24562 |
| Pettersen | Kari | 77895 |
| Pettersen | Kari | 44678 |
| | | 34764 |
The RIGHT JOIN keyword returns all the rows from the right table (Orders), even if there are no matches in the left table (Persons).
SQL FULL JOIN Keyword
The FULL JOIN keyword return rows when there is a match in one of the tables.
SQL FULL JOIN Syntax
| SELECT column_name(s) FROM table_name1 FULL JOIN table_name2 ON table_name1.column_name=table_name2.column_name |
SQL FULL JOIN Example
The "Persons" table:
| P_Id | LastName | FirstName | Address | City |
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
The "Orders" table:
| O_Id | OrderNo | P_Id |
| 1 | 77895 | 3 |
| 2 | 44678 | 3 |
| 3 | 22456 | 1 |
| 4 | 24562 | 1 |
| 5 | 34764 | 15 |
Now we want to list all the persons and their orders, and all the orders with their persons.
We use the following SELECT statement:
| SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo FROM Persons FULL JOIN Orders ON Persons.P_Id=Orders.P_Id ORDER BY Persons.LastName |
The result-set will look like this:
| LastName | FirstName | OrderNo |
| Hansen | Ola | 22456 |
| Hansen | Ola | 24562 |
| Pettersen | Kari | 77895 |
| Pettersen | Kari | 44678 |
| Svendson | Tove | |
| | | 34764 |
The FULL JOIN keyword returns all the rows from the left table (Persons), and all the rows from the right table (Orders). If there are rows in "Persons" that do not have matches in "Orders", or if there are rows in "Orders" that do not have matches in "Persons", those rows will be listed as well
The SQL UNION operator combines two or more SELECT statements.
The SQL UNION Operator
The UNION operator is used to combine the result-set of two or more SELECT statements.
Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.
SQL UNION Syntax
| SELECT column_name(s) FROM table_name1 UNION SELECT column_name(s) FROM table_name2 |
Note: The UNION operator selects only distinct values by default. To allow duplicate values, use UNION ALL.
SQL UNION ALL Syntax
| SELECT column_name(s) FROM table_name1 UNION ALL SELECT column_name(s) FROM table_name2 |
PS: The column names in the result-set of a UNION are always equal to the column names in the first SELECT statement in the UNION.
SQL UNION Example
Look at the following tables:
"Employees_Norway":
| E_ID | E_Name |
| 01 | Hansen, Ola |
| 02 | Svendson, Tove |
| 03 | Svendson, Stephen |
| 04 | Pettersen, Kari |
"Employees_USA":
| E_ID | E_Name |
| 01 | Turner, Sally |
| 02 | Kent, Clark |
| 03 | Svendson, Stephen |
| 04 | Scott, Stephen |
Now we want to list all the different employees in Norway and USA.
We use the following SELECT statement:
| SELECT E_Name FROM Employees_Norway UNION SELECT E_Name FROM Employees_USA |
The result-set will look like this:
| E_Name |
| Hansen, Ola |
| Svendson, Tove |
| Svendson, Stephen |
| Pettersen, Kari |
| Turner, Sally |
| Kent, Clark |
| Scott, Stephen |
Note: This command cannot be used to list all employees in Norway and USA. In the example above we have two employees with equal names, and only one of them will be listed. The UNION command selects only distinct values.
SQL UNION ALL Example
Now we want to list all employees in Norway and USA:
| SELECT E_Name FROM Employees_Norway UNION ALL SELECT E_Name FROM Employees_USA |
Result
| E_Name |
| Hansen, Ola |
| Svendson, Tove |
| Svendson, Stephen |
| Pettersen, Kari |
| Turner, Sally |
| Kent, Clark |
| Svendson, Stephen |
| Scott, Stephen |
The SQL SELECT INTO statement can be used to create backup copies of tables.
The SQL SELECT INTO Statement
The SELECT INTO statement selects data from one table and inserts it into a different table.
The SELECT INTO statement is most often used to create backup copies of tables.
SQL SELECT INTO Syntax
We can select all columns into the new table:
| SELECT * INTO new_table_name [IN externaldatabase] FROM old_tablename |
Or we can select only the columns we want into the new table:
| SELECT column_name(s) INTO new_table_name [IN externaldatabase] FROM old_tablename |
SQL SELECT INTO Example
Make a Backup Copy - Now we want to make an exact copy of the data in our "Persons" table.
We use the following SQL statement:
| SELECT * INTO Persons_Backup FROM Persons |
We can also use the IN clause to copy the table into another database:
| SELECT * INTO Persons_Backup IN 'Backup.mdb' FROM Persons |
We can also copy only a few fields into the new table:
| SELECT LastName,FirstName INTO Persons_Backup FROM Persons |
SQL SELECT INTO - With a WHERE Clause
We can also add a WHERE clause.
The following SQL statement creates a "Persons_Backup" table with only the persons who lives in the city "Sandnes":
| SELECT LastName,Firstname INTO Persons_Backup FROM Persons WHERE City='Sandnes' |
SQL SELECT INTO - Joined Tables
Selecting data from more than one table is also possible.
The following example creates a "Persons_Order_Backup" table contains data from the two tables "Persons" and "Orders":
| SELECT Persons.LastName,Orders.OrderNo INTO Persons_Order_Backup FROM Persons INNER JOIN Orders ON Persons.P_Id=Orders.P_Id |
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