Which SQL function replaces NULL values in a column with values from another column during data migration?

• A. IFNULL
• B. COALESCE
• C. NULLIF
• D. REPLACE

Answer: (B)

Handling NULLs by picking the first non-NULL value across candidates is the key idea. COALESCE does exactly that: it looks through its arguments from left to right and returns the first value that is not NULL. This makes it perfect for data migration when you want to keep an existing value if present, but fall back to another column’s value if the original is NULL. For example, you can use COALESCE(target_col, source_col) to produce a column that preserves target_col when it has data, and uses source_col only when target_col is NULL. COALESCE also supports more than two arguments, which is handy if you have multiple fallback columns. By contrast, the other options don’t fit as well: IFNULL can work in some databases but is not as portable and typically handles only two values; NULLIF turns matching values into NULL rather than substituting from another column; REPLACE operates on strings and doesn’t address NULL substitution.