Boolean Operators: Parentheses

You can use parentheses to build a search with a combination of Boolean Operators.

Take a look at the illustrations below - the search terms and Boolean Operators are identical, but the parentheses create two very different searches.

Using parentheses in your Boolean search, is a lot like using them in arithmetic - the search inside the parentheses is done first.

Lets take a closer look.

The blue area in the first illustration represents the results that would be returned for the search (Cetacean OR Whale) AND Pacific, which yields 633 results and gives us articles with the following combinations:

Cetacean, Pacific
Whale, Pacific
Cetacean, Whale, Pacific

Note that Cetacean and Whale may or may not be present, but Pacific will appear in every search result.  This is because Cetacean and Whale are linked with OR while Pacific is added to the search with AND.

The blue area in the second illustration represents the results that would be returned for the search Cetacean OR (Whale AND Pacific), which yields 1718 results and gives us articles with the following combinations:

Whale, Pacific
Whale, Pacific, Cetacean
Cetacean

Note that Whale and Pacific must always appear together, or not at all, while Cetacean may be the only term present.  This is because Whale and Pacific are linked with AND and Cetacean is added to the search with OR.

When the same search is run with no parentheses - Cetacean OR Whale AND Pacific - we get the same search results as Cetacean OR (Whale AND Pacific). This is because of search precedence which is discussed to the right.

Here are a few more examples of using parentheses with Boolean Operators:

• (children OR kids) AND nutrition
  Notice that the OR links two synonyms


• (elderly OR "older adults") AND (computers OR technology)
  You can use two or more sets of parentheses


• ((teenager OR teen OR adolescent) AND media) NOT television
  You can also use parentheses within parentheses. 

Databases are programmed to prioritize Boolean Operators.  This is often called "search precedence" and it influences how your search is performed. 

For instance, most databases give AND precedence over OR, meaning terms linked with AND will be searched before those linked with OR, no matter the order they appear in your search. 

In any database, using parentheses will override the search precedence.