Both approaches, however, can yield identical solutions. My preference is to use common table expressions ( CTEs) rather than nested subqueries - CTEs allow for a more linear illustration of the data wrangling sequence. Keep in mind there is usually more than one way to obtain the correct answer to a SQL problem. The questions outlined below include example solutions confirmed to work in PostgreSQL. I’ve found this to be a more satisfying learning experience. Locally executing a query, on the other hand, is instantaneous and allows for rapid iteration through syntax bugs and intermediate tables. In addition, running SQL queries in your browser can be extremely slow - the data sets are large and retrieval speed is often throttled for non-premium users. LeetCode, for instance, doesn’t support the use of window functions and hides its most interesting questions behind a paywall. Web-based SQL training resources fall short along a few dimensions. You can query against the input table using PSequel (shown above) and easily construct new tables for your own problems using this template. The first block of text in each query shown below establishes the input table and follows the format: WITH input_table (column_1, column_2) AS (VALUES (1, 'A'), (2, 'B')) Try these queries yourself using PSequel and the input tables provided below. PSequel is only available for Mac - if you’re using a PC, you can try one of these Windows alternatives. You can try these out yourself by downloading PostgreSQL and PSequel (see this tutorial for a step-by-step installation guide) and then running the queries shown in the grey boxes in the text below.
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