SQL query - What does taking the MIN of this boolean expression mean?

Question

excuse my ignorance about this... I'm taking a data analysis course and I stumbled upon this query in an exercise:

SELECT
  CASE
    WHEN MIN(REGEXP_CONTAINS(STRING(ActivityDate), DATE_REGEX)) = TRUE THEN
  "Valid"
  ELSE
    "Not Valid"
  END
    AS valid_test
  FROM
   `tracker_data_clean.daily-activity-clean`;

ActivityDate is a field that contains date type data and DATE_REGEX is a regular expression variable for a date format string.

What I don't know, is what does taking the MIN() of this boolean expression REGEX_CONTAINS do or mean.

I would appreciate if any of you could help me understand the concept of doing this.

Thanks !

Answer 1

The query selects rows from the table and applies the REGEXP_CONTAINS() function to every (string-converted) value in the ActivityDate column. REGEXP_CONTAINS() will either return true or false based on whether the value matches the regex pattern in DATE_REGEX .

How MIN() behaves here can vary by implementation:

• Booleans might be coerced as integers, so MIN() is evaluating 0 's and 1 's. If all the values are 1 ( true ), MIN() will be 1 ( true ), otherwise it will be 0 ( false ).
• Other implementations might evaluate booleans directly, so MIN() returns true if all of the values are true , because the minimum value is true ( true being "greater" than false ), otherwise it returns false .

The result, based on the implementation, is that MIN() returns 0 / 1 , or false / true . Either way, that result is compared to true in the CASE statement. If all values matched the regex, the comparison will be true .

Basically, the query is "does every row have a valid date in the ActivityDate column?" The result will be a table with a single column valid_test and one row, containing "Valid" if they all match, "Not Valid" otherwise.

Another way to look at it that would be relatable to some programming languages is that MIN(bool_function()) is analogous to all() , meaning return true if all values are true . Similarly, MAX(bool_function()) would be analogous to any() , meaning return true if any value is true .