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Advanced Searching: Building Your Search

Boolean Logic

The term Boolean comes from the name of its originator George Boole (1815-1864). He developed a system of logic that now underpins the workings of modern computers. Boolean logic enables you to retrieve information from computers by expressing the relationships between words and phrases using language. Boolean Logic has three components AND, OR and NOT.

Boolean AND
AND links concepts together. A database searches for results that contain Concept 1 and Concept 2, then compares the results to display only those results that contain both concepts. This can be represented using the Venn diagram right. The area in the centre where the circles cross represents the results that contain both terms.
Boolean OR
OR combines concepts together. The database locates all the results that contain concept 1 and all the results that contain concept 2 then combines both groups of results into a single set. Each circle represents the results that contain the words humour and the results that contain the word comedy. Using OR retrieves all the results and these are represented by the total area of both circles.
Boolean NOT
NOT excludes results that contain concepts that you do not want to find. Searching for advertising NOT marketing retrieves all the results that contain the word advertising, but excludes the results that contain the word marketing even if the word advertising is present. Each circle represents the results that contain the words advertising and the results that contain the word marketing. Using NOT only displays results containing the word advertising represented by half-moon shape of the advertising circle.

Combining different Boolean Operators

Once you understand how Boolean operators work you need to go one step further and understand what happens when you combine more than one Boolean operator in the same search.

Computers process the AND operator first and then the OR operator

If you type: Homeless AND Teenage OR Adolescent

It will do this:   Homeless AND Teenage    OR Adolescent

You need to say:  Homeless AND    (Teenage OR Adolescent)

Group the OR concepts together using ( ) to ensure that the search is processed in the expected way.

 Search interfaces are usually designed to help you create logical searches that work. Where an interface is very simple or where there is an Expert Search option you may wish to use ( ) to group OR terms together.

Typically search interfaces offer a layout similar to mySearch, with a series of text boxes to enter keywords combined with dropdown menus to select various Boolean Operators.

Wildcards

Some terms present problems because letters within them change depending on the context e.g. UK and US spelling differences or plurals. An asterisk or hash or question mark can sometimes be used in the middle of a word to indicate that a letter or letters may change, or an extra letter may be introduced. For example:

•Wom*n will find woman or women
•C*liac will find both American and English spellings celiac and coeliac

Check specific database Help sections to find which wildcard terms are used. 

Truncation

Many terms in English have more than one ending depending on the context. When searching databases only the words you type in are retrieved. A complete search requires all the words with all possible endings to be included.

 You can achieve this by truncating the word using an asterisk:

  • Advert* - will find advertising, adverts, advertisement, advertisements, advertisers, advertise
  • Child* - will find child, children, childhood

 However beware of over-truncation:

 Diet* - will find diet, diets, dietary, dieted, BUT it will also find diethyl