After identifying your keywords, you need to define the relationship between them using the Boolean operators OR, AND and NOT. All databases, and even Google, use Boolean operators to set up a flexible search.

OR » finds records that contain ANY of the terms
When looking for references that contain the term 'microorganisms' or an alternative term like 'microbes', use the OR operator to define the relationship between the terms. ie. microorganisms OR microbes OR is used for alternative or synonymous terms. Using the OR operator broadens your search. The more alternative terms you use, the larger the number of references you retrieve.
AND » finds records that contain BOTH terms
When looking for references about 'salinity' in 'Australia', use the AND operator to specify that both terms must appear in the references retrieved. ie. salinity AND Australia When you join two words or phrases together with AND, the database will retrieve only those records which contain both terms. This narrows your search.
NOT » finds records with the first search term, but not the second
Use the NOT operator to exclude a term. For example, greenhouse NOT glasshouse   WARNING - use NOT with care; relevant articles can be excluded if they briefly mention the second term.

In some databases the proximity between keywords can be specified using operators like NEAR, WITH, or PRECEDES

These can specify that your keywords appear in the same sentence or field, or that a search on a phrase will find the words in any order.

Proximity operators vary from database to database - check the help page within each database.

Some examples:

Web of Science: NEAR/x where x is the maximum number of words that separate the terms, which can appear in any order,

Scopus: PRE/x 'precedes by' or W/x 'within x words of each other'. PRE/x specifies the exact order of terms. W/x does not - the terms can appear in any order.

Phrase searching

 To specify that two or more words must appear as a phrase, use double quotes.

• For example, “global warming”.

Truncation

To search for alternative endings of words, use truncation or stem searching.  In most databases the truncation operator is an asterisk ( * ).

• For example, a search for comput* will retrieve computer, computers, computation, computing.

Wildcards

Sometimes there are slight differences in the spelling of a word, English and American spelling being an example. To facilitate searching on alternative spellings, some databases allow wildcard searching. The wildcard operator is often a question mark ( ?) or an asterisk (*).

• For example,  colo?r will find color or colour.

Note: Truncation and wildcard symbols can vary from database to database - check the help page of each database.