Having chosen your search terms, you next need to think about writing a search strategy to combine them in the most effective way. There are three main commands used to do this: AND, OR and NOT. These are sometimes referred to as 'boolean operators'. Instead of searching for them as words, the database will interpret them as a command.
AND is used to combine different topic words so that all of the words must appear in the search results.
Add more keywords using AND to narrow down your search and find fewer results. For example: 
Sunscreen AND Skin
Child AND Psychology
OR is used to combine similar topic words so that all of the words can appear in the search results.
Add more keywords using OR to broaden your search and find more results. For example: 
Drug OR Medicine
Football OR Soccer
When using both 'AND' and 'OR in your search strategy, it is important to get them in the right order! If you seach for:
Pregnancy AND Drugs OR Medicines
You will find articles covering pregnancy and drugs, but then everything in the database on medicines regardless of whether it is connected with pregnancy. To avoid this, use brackets (parentheses):
Pregnancy AND (Drugs OR Medicines)
This tells the computer to combine your synonyms (drugs/medicines) using 'OR' first, then combine with pregnancy.
Most databases allow you to use NOT to exclude a term that you don't want to appear. For example: 
Zebra NOT Fish
But this should be used with caution. For example:
Microgeneration AND power NOT USA
Would exclude articles comparing microgeneration of power in the USA with the rest of the world.
