Explain CASE, COALESCE, NULLIF, and the vendor equivalents IFNULL / NVL / ISNULL.

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These are SQL's conditional expressions. CASE is the standard, general-purpose if/else; COALESCE returns the first non-NULL argument; NULLIF(a, b) returns NULL when a = b. All three are ANSI standard and portable — the vendor functions IFNULL, NVL, and ISNULL are proprietary two-…