Entity Framework Core 2.0.2中StartsWith生成的SQL条件是否冗余？

Great question—let’s break this down clearly, since it ties into a key quirk of SQL string comparisons vs. .NET’s behavior.

Are the Two Conditions Identical? #

No, they aren’t fully equivalent in all scenarios. Both aim to check if Name starts with your search term, but they handle edge cases differently:

• [t].[Name] LIKE @__term_1 + N'%': This is the standard SQL prefix match. It works for most cases, but SQL Server (and some other databases) ignores trailing spaces in the search term when using LIKE. For example, if your @__term_1 is 'a ' (with a trailing space), this condition would match 'a' (since the trailing space is treated as non-existent in the LIKE comparison).
• LEFT([t].[Name], LEN(@__term_1)) = @__term_1: This explicitly checks that the first LEN(@__term_1) characters of Name exactly equal the search term—including any trailing spaces. So if @__term_1 is 'a ', this only returns rows where the first two characters of Name are 'a ', which matches how .NET’s StartsWith works (where "a".StartsWith("a ") returns false).

Can You Use Just One? #

It depends on your needs:

• If you never use search terms with trailing spaces or don’t care about matching .NET’s exact behavior, using just the LIKE condition will work for most cases and is slightly simpler. It also has potential performance benefits, as LIKE prefix matches can leverage indexed columns more efficiently than LEFT in some scenarios.
• If you need your query to align perfectly with .NET’s StartsWith semantics (especially handling trailing spaces correctly), you need both conditions. Using only the LEFT condition would also work for exact prefix matching, but it might trigger full table scans on large datasets if your Name column isn’t indexed optimally.

EF Core includes both conditions by default to eliminate the mismatch between SQL’s string comparison rules and .NET’s expected behavior, preventing unexpected results for edge cases.

内容的提问来源于stack exchange，提问作者Serg