## Syntax for Entrez Searching

Entrez databases use a search syntax closer to natural language than does z39.50 databases. Enter key words, and the Entrez server will generate a custom set of search results based on its retrieval algorithms. Note that when searching PubMed for personal author names, ``,'` should not be entered between first names and last initials, and that first initials (as opposed to first names) are typically used, as in ``wright s'` or ``fisher ra'`.

For more focused searching, Entrez allows terms to be combined using Boolean operators. According to the Entrez documentation, available at Boolean Operators, these must be entered as follows.

``AND:'` | Combine two search terms together to find all documents that contain BOTH terms.
``OR:'` | Combine two search terms together to find all documents that contain EITHER term.
``NOT:'` | Combine two search terms together to find all documents that contain search term 1 BUT NOT search term 2.

The Entrez search rules and syntax for using Boolean operators are:

Another way in which searches can be focused in PubMed is by the use of extensions to the search terms. For instance, to search for ``random genetic drift'` in the MeSH terms field only, enter the following:

    random genetic drift [hm]

into the search field. Note that the extension appears in brackets after the search key (``random genetic drift,'` in this case).

Other useful extensions include the following.

Extension | Meaning
--- | ---
au | Author
1au | First author
lastau | Last author name
la | Language (use the first three letters of the language's name, but note that "ja" means "japanese".)
sh | MeSH Subheadings
dp | Publication date in YYYY/MM/DD format. Month and day are optional.
ti | Title
tiab | Title or abstract

Complete documentation of the extensions is provided at Search Field Description and Tags.