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DP-600 · 2.2.7 · Prepare data

Duplicates and NULLs

Identify and resolve duplicate data, missing data, or null values

intentionally, replace missing values only when a meaningful default exists, and test with IS NULL or IS NOT NULL.

The core idea

Data-quality infographic for duplicates, missing values, and NULL logic using DISTINCT, ROW_NUMBER, COALESCE, and IS NULL.

Profile data before fixing it. Exact duplicate rows can be removed directly, while duplicate business keys often need a rule such as keeping the latest row with ROW_NUMBER.

means unknown, not zero or an empty string. can replace it when a business-valid default exists.

Do not compare a value with = NULL. Use IS NULL or IS NOT NULL.

Key points

  • Exact duplicates: DISTINCT or Remove duplicates
  • Duplicate keys: rank rows and keep the intended record
  • Missing values: only when the default is meaningful
  • : use IS NULL or IS NOT NULL

Click an underlined term for a one-line explanation, or see them all in the glossary .

Exam trap

WHERE Phone = never returns a row, because a comparison with NULL is never true. Use IS NULL.

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