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NULL in SQL represents the absence of a value, distinct from an empty string or zero. Key points include: NULL cannot be tested in a list, comparisons like NULL = NULL are not allowed, COUNT(column) excludes NULLs while COUNT(*) includes them, aggregate functions ignore NULLs, ORDER BY places NULLs first by default, and joins do not match NULLs. Handling NULLs can be done using IS NULL, COALESCE, IFNULL/ISNULL, and NULLIF.
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In SQL, WHERE and HAVING differ in their application timing. WHERE is used before aggregation, while HAVING is used after. If 'quantity' is filtered in the HAVING clause, it refers to the SUM of quantities per country bucket, not the original 'quantity'. It's suggested to rename the aggregated column (e.g., 'total_quantity') for clarity. The choice between WHERE and HAVING depends on the need for filtering before or after aggregation.
10 months ago
The DECLARE command in SQL can simplify queries with varying parameters. Instead of repeatedly using a WHERE statement for different time periods, you can declare variables before your query and adjust them for the desired timeframe. For instance, you can assign names to your variables along with their respective data types, such as '@start_date' and '@end_date' storing 'date' data type.
