How to extract ISO week numbers from dates in ClickHouse ®

ISO week numbers are crucial for consistent analytics across year boundaries, avoiding partial-week aggregates in reporting. The toISOWeek() function in ClickHouse®, adhering to ISO 8601 standards, starts weeks on Monday, with week 1 containing January 4th. This can lead to counterintuitive results, such as December 31st being part of week 1 of the following year, which can confuse developers working with time-based analytics. The function, available since ClickHouse® version 19.15, takes a date or datetime value and returns a week number between 1 and 53. It supports various data types and automatically handles time zones. Developers can use it alongside toISOYear() for comprehensive year-week analysis, especially important around year boundaries. The guide provides strategies for optimizing performance, such as using materialized columns and secondary indexes, and explains how to build robust analytics APIs using Tinybird's managed ClickHouse® platform, which simplifies infrastructure management and ensures consistent ISO week calculations across regions and time zones.