How to compare strings in SQL ignoring case?

Matching with wildcards

In case you need to achieve partial matching, use LIKE operator alongside the LOWER() or UPER() function:

SELECT * FROM my_table WHERE LOWER(column_name) LIKE LOWER('%my_string%');
-- Who cares about the case when you've got wildcards?
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Applying global session settings

For Oracle databases, you can universally set case-insensitive comparisons for an entire session:

ALTER SESSION SET NLS_COMP = LINGUISTIC;
ALTER SESSION SET NLS_SORT = BINARY_CI;
-- Hey Oracle, take a chill pill on case sensitivity, will ya!
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These commands help in reducing redundancies in your queries when case insensitivity is consistently the need of the hour.

Considering efficiency and special characters

Utilizing LIKE with a prefix % can potentially slow down your query. For optimized performance, if case-insensitive searches are frequent, create a functional index on the LOWER() or UPER() function:

CREATE INDEX IDX_LOWER_COLUMN ON my_table ((LOWER(column_name)));
-- Say hello to my little 'index' friend!
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Remember when dealing with non-English characters, default case conversion functions might not apply predictably. Hence examination of collation and locale-specific settings is recommended.

Index creation for case-insensitivity

Setting up the indexes to facilitate case-insensitive searching can lend a hand in enhancing the query efficiency. Here's an example for PostgreSQL:

CREATE INDEX idx_case_insensitive_column 
ON table_name ((LOWER(column_name)));
-- Let the index do the heavy lifting
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Align your queries using the LOWER() function to fully utilize the index created.

Addressing compatibility with non-English letters

Issues may arise when dealing with non-English characters since simple case conversion isn't always universal. In such cases, delve into the collation and encoding settings:

SELECT * FROM my_table WHERE column_name ILIKE 'my_string';
-- Because 'string' is an international citizen
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The above instance employs PostgreSQL's ILIKE, performing a case-insensitive search, considering locale-specific variations.