The NLS_COMP parameter controls how Oracle Database performs string comparison operations in SQL statements. This parameter works in conjunction with NLS_SORT to determine whether character comparisons use binary values or linguistic rules.

The NLS_COMP parameter affects all comparison operations including WHERE clauses, JOIN conditions, DISTINCT operations, GROUP BY clauses, and ORDER BY statements. When set to BINARY, Oracle ignores the NLS_SORT setting and performs direct binary comparisons. When set to LINGUISTIC, Oracle uses the collation rules specified by the NLS_SORT parameter to perform linguistic appropriate comparisons.

For detailed explanation of NLS_SORT parameter values, see NLS_SORT Parameter
For indexing strategies with this parameter, see NLSSORT Function Indexes

The NLS_COMP parameter can be modified at the session level to control comparison behavior for the current database session.

ALTER SESSION SET NLS_COMP = { BINARY | LINGUISTIC | ANSI };

• BINARY (Default value)
  SQL operations (sorting and comparison) are based on the binary values of characters, regardless of the value set for NLS_SORT. This provides the fastest performance as no linguistic processing is required. Under BINARY comparison, characters are compared using their numeric representation in the database character set, making comparisons accent-sensitive and case-sensitive. For example, 'A' and 'a' are treated as completely different characters.
  
  
• LINGUISTIC
  SQL operations (sorting and comparison) are based on linguistic rules according to the setting of NLS_SORT. This enables linguistic appropriate comparisons that can be accent-insensitive, case-insensitive, or both, depending on the NLS_SORT value. When LINGUISTIC is enabled, Oracle applies the transformation rules specified by NLS_SORT to both sides of comparison operations. To improve performance, you can create a linguistic index on the column where linguistic comparisons will be performed.
  
  
• ANSI
  Deprecated. No longer recommended for new applications. Use LINGUISTIC instead.
  

The choice between BINARY and LINGUISTIC represents a trade-off between performance and linguistic accuracy. BINARY comparisons are faster but may not provide the user-friendly search behavior expected in international applications.

For more information about NLS_COMP values and their impact on SQL sorting/comparison, click the following link: Table 5-2 Linguistic Comparison Behavior with NLS_COMP Settings