Boolean operators connect your search words together to either narrow or broaden your set of results.
The three basic boolean operators are: AND, OR, and NOT.

AND

Use AND in a search to:

• narrow your results
• tell the database that ALL search terms must be present in the resulting records
• example: aphasia AND stroke AND depression

The darkest area in the middle of the Venn diagram below represents the result set for this search. It is a small set using AND, the combination of all three search words.

OR

Use OR in a search to:

• connect two or more similar concepts (synonyms)
• broaden your results
• tell the database that ANY of your search terms can be present in the resulting records
• example: treatment OR therapy

Both circles represent the result set for this search. It is a big set because both of those words are valid using the OR operator.

NOT

Use NOT in a search to:

• exclude words from your search
• narrow your search, telling the database to ignore concepts that may be implied by your search terms
• example: communication disorders NOT aphasia

Only a portion of the blue circle on the left represents the result set for this search.

Be aware:  In some databases, the AND is implied. 

Databases usually recognize AND as the primary operator, and will connect concepts with AND together first.
If you use a combination of AND and OR operators in a search, enclose the words to be "ORed" together in parentheses.

• For example: aphasia AND (treatment OR therapy)

Note: Boolean operators can be case sensitive and in some databases should appear in all capital letters.

Use truncation and wildcards for ​root words that have multiple endings and for words that are spelled differently, but mean the same thing.  
Truncation/wildcard symbols vary by database.  Check the help screens to find out which symbols are used.

Truncation

Truncation, also called stemming, is a technique that broadens your search to include various word endings and spellings.

To use truncation, enter the root of a word and put the truncation symbol at the end.
The database will return results that include any ending of that root word.
Examples:
child* = child, childs, children, childrens, childhood
genetic* = genetic, genetics, genetically
Truncation symbols may vary by database; common symbols include: * and ?

Wildcards

Similar to truncation, wildcards substitute a symbol for one letter of a word.

This is useful if a word is spelled in different ways, but still has the same meaning.
Examples:
wom!n = woman, women
p?ediatric = paediatric, pediatric 

Wildcard symbols may vary by database; common symbols include: ? and !

Phrase searching

Most databases allow you to specify that adjacent words be searched as phrases. Using quotation marks around search words is a common way to do phrase searching, but not all databases or search engines use them. Put a phrase in quotation marks when you want to find words in the exact order. 

Example:  "quality of life"

Proximity operators

Proximity operators also vary by database, but common ones include:
N, N#, NEAR, or NEAR# 

• Near specifies that the words may appear in any order.
• Substitute the # with a number of words that may appear in between.
• Example:   cloning n3 human = cloning of humans, human cloning, etc.

Consult the database Help screens to find out how to search by phrases or to specify proximity.

Stop words are frequently occurring, insignificant words that appear in every database record, article or web page.Try to eliminate stop words from your database search queries, because:

• Many databases ignore common words from your search statement. 
• If included, the database returns far too many results.

Common stop words include: a, an, the, in, of, on, are, be, if, into, which​

Make sure stop words are included only if they are a significant part of your search. For example: "quality of life"

Some databases use controlled vocabularies (in other words, they assign subject headings from a pre-defined set of terms to every item) and other databases operate only using keywords or natural language. If a database has a controlled vocabulary, you can use the subject headings to find relevant items on the same topic. Searching by subject headings is the most precise way to search article databases.

Look to see if the database has a thesaurus to browse for subjects that match your topic (check the Help screens).

Another way to find subject headings:

Start with a keyword search, using words/phrases that describe your topic.
Browse the results; choose 2 or 3 that are relevant.
Look at the Subject or Descriptor field and note the terms used.
Redo your search using those terms.
Your results will be more precise than your initial keyword search.