LINQ in Entity Framework

To retrieve lists from database you can use IEnumerbale<T> or IQueryable<T> interfaces. IQueryable allows you to do LINQ-to-SQL while IEnumerable<T> is LINQ-to-Objects. Both gives you the opportunity to do deferred execution. (Read this article for more).

In IEnumerable all objects matching the original query will be loaded to the memory from database. 

Above first and last LINQ statements will generate first SQL query while second LINQ expression will generate the second query below,

LINQ First and FirstOrDefault

LINQ Single and SingleOrDefault

LINQ Any

Note that Count will iterate all the records in the table while Any will stop at first record it finds. (source)

LINQ Count and LongCount

The difference between Count and Count_Big is latter returns bigint while count returns int. See more details on data types.

LINQ Contains

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