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A SOQL query is the equivalent of a SELECT SQL statement and searches the org database. SOSL is a programmatic way of performing a text-based search against the search index.

Whether you use SOQL or SOSL depends on whether you know which objects or fields you want to search, plus other considerations.

Use SOQL when you know which objects the data resides in, and you want to:

Retrieve data from a single object or from multiple objects that are related to one another.

Count the number of records that meet specified criteria.

Sort results as part of the query.

Retrieve data from number, date, or checkbox fields.