Deduplicate CRM Contacts

SQL practice question · Hard · Data Cleaning · Accenture, Deloitte

A client migration needs one row per real person. Clean email = LOWER(TRIM(email)), treated as missing when NULL or blank. Clean phone = the digits of phone only (drop spaces, '+', '-', '(', ')', '.'), keeping the LAST 10 digits; missing when no digits. Identity key: the clean email if present; otherwise the clean phone; otherwise the contact is unique on its own. Within each identity keep the row with the latest updated_at (tie: larger contact_id). Return contact_id, full_name (TRIM), clean_email and clean_phone (NULL when missing). Order by contact_id.

Table: crm_contacts

Schema of crm_contacts
ColumnType
contact_idINTEGER
full_nameTEXT
emailTEXT
phoneTEXT
updated_atTIMESTAMP
Sample rows of crm_contacts
contact_idfull_nameemailphoneupdated_at
1Rohit SharmaRohit.S@Mail.com+91 98290-111112026-01-10 10:00:00
2Rohit Srohit.s@mail.com 98290111112026-02-01 09:00:00
3Neha GuptaNULL(982) 902-22222026-01-05 08:00:00
4Neha G.+91-98290222222026-01-20 08:00:00
5Arjun Raoarjun@corp.in98290333332026-01-15 12:00:00
6Meena IyerNULLNULL2026-01-01 00:00:00
Expected output on the sample data
contact_idfull_nameclean_emailclean_phone
2Rohit Srohit.s@mail.com9829011111
4Neha G.NULL9829022222
5Arjun Raoarjun@corp.in9829033333
6Meena IyerNULLNULL

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