Filtering

WHERE decides which rows are eligible for everything that comes after. If a filter is wrong, every downstream number is wrong, even when the...

Most real filters are multi-part: paid orders from this quarter, customers in these three countries but not trial accounts, etc. AND, OR, an...

Once you have written a few WHERE clauses, two patterns show up over and over: "is this value one of a list?" and "is this value in a range?...

= checks for exact equality. LIKE lets you match text by pattern: prefixes, suffixes, substrings, and single-character gaps. It is how you a...

NULL means missing or unknown. It is not zero. It is not an empty string. It is not false. It is the absence of a value, and SQL treats it d...