SQL Union & Intersect Command

SQL UNION Query

The SQL UNION query allows you to combine the result sets of 2 or more SQL SELECT statements. It removes duplicate rows between the various SELECT statements.

Each SQL SELECT statement within the UNION query must have the same number of fields in the result sets with similar data types.

The syntax for the SQL UNION query is:

select field1, field2, . field_n

from tables

UNION

select field1, field2, . field_n

from tables;

SQL UNION Query - Returns single field example

The following is an example of the SQL UNION query that returns one field from multiple SELECT statements (and both fields have the same data type):

select supplier_id

from suppliers

UNION

select supplier_id

from orders;

In this SQL UNION query example, if a supplier_id appeared in both the suppliers and orders table, it would appear once in your result set. The SQL UNION query removes duplicates. If you do not wish to remove duplicates, try using the SQL UNION ALL query.

SQL UNION Query - Using SQL ORDER BY Clause example

The SQL UNION query can use the SQL ORDER BY clause to order the results of the query.

For example:

select supplier_id, supplier_name

from suppliers

where supplier_id > 2000

UNION

select company_id, company_name

from companies

where company_id > 1000

ORDER BY 2;

In this SQL UNION query, since the column names are different between the two SQL SELECT statements, it is more advantageous to reference the columns in the SQL ORDER BY clause by their position in the result set. In this example, we've sorted the results by supplier_name / company_name in ascending order, as denoted by the "ORDER BY 2".

The supplier_name / company_name fields are in position #2 in the result set.

SQL INTERSECT Query

The SQL INTERSECT query allows you to return the results of 2 or more "select" queries. However, it only returns the rows selected by all queries. If a record exists in one query and not in the other, it will be omitted from the INTERSECT results.

Each SQL statement within the SQL INTERSECT query must have the same number of fields in the result sets with similar data types.

The syntax for the SQL INTERSECT query is:

select field1, field2, . field_n

from tables

INTERSECT

select field1, field2, . field_n

from tables;

SQL INTERSECT Query - Single field example

The following is an example of an SQL INTERSECT query that has one field with the same data type:

select supplier_id

from suppliers

INTERSECT

select supplier_id

from orders;

In this SQL INTERSECT query example, if a supplier_id appeared in both the suppliers and orders table, it would appear in your result set.

SQL INTERSECT Query - Using ORDER BY Clause example

The following is an SQL INTERSECT query that uses an SQL ORDER BY clause:

select supplier_id, supplier_name

from suppliers

where supplier_id > 2000

INTERSECT

select company_id, company_name

from companies

where company_id > 1000

ORDER BY 2;

Since the column names are different between the two "select" statements, it is more advantageous to reference the columns in the SQL ORDER BY clause by their position in the result set. In this example, we've sorted the results by supplier_name / company_name in ascending order, as denoted by the "ORDER BY 2".

The supplier_name / company_name fields are in position #2 in the result set

SQL Having Clause

SQL: HAVING Clause

The SQL HAVING clause is used in combination with the SQL GROUP BY clause. It can be used in an SQL SELECT statement to filter the records that a SQL GROUP BY returns.

The syntax for the SQL HAVING clause is:

SELECT column1, column2, ... column_n, aggregate_function (expression)

FROM tables

WHERE predicates

GROUP BY column1, column2, ... column_n

HAVING condition1 ... condition_n;

aggregate_function can be a function such as SQL SUM function, SQL COUNT function, SQL MIN function, or SQL MAX function.

SQL HAVING Clause - Using the SUM function example

You could also use the SQL SUM function to return the name of the department and the total sales (in the associated department). The SQL HAVING clause will filter the results so that only departments with sales greater than $1000 will be returned.

SELECT department, SUM(sales) as "Total sales"

FROM order_details

GROUP BY department

HAVING SUM(sales) > 1000;

SQL HAVING Clause - Using the COUNT function example

You could use the SQL COUNT function to return the name of the department and the number of employees (in the associated department) that make over $25,000 / year. The SQL HAVING clause will filter the results so that only departments with more than 10 employees will be returned.

SELECT department, COUNT(*) as "Number of employees"

FROM employees

WHERE salary > 25000

GROUP BY department

HAVING COUNT(*) > 10;

SQL HAVING Clause - Using the MIN function example

You could also use the SQL MIN function to return the name of each department and the minimum salary in the department. The SQL HAVING clause will return only those departments where the starting salary is $35,000.

SELECT department, MIN(salary) as "Lowest salary"

FROM employees

GROUP BY department

HAVING MIN(salary) = 35000;

SQL HAVING Clause - Using the MAX function

For example, you could also use the SQL MAX function to return the name of each department and the maximum salary in the department. The SQL HAVING clause will return only those departments whose maximum salary is less than $50,000.

SELECT department, MAX(salary) as "Highest salary"

FROM employees

GROUP BY department

HAVING MAX(salary) < 50000;