4 Solutions: UNION ALL, UNION, FROM, CASE
SOLUTION-1: \nUNION ALL\n\n# EXPLANATION\n+ We use three separate SELECT statements to extract the data for each store, and use UNION ALL to concatenate the res
# SOLUTION-1: \nUNION ALL\n\n# EXPLANATION\n+ We use three separate SELECT statements to extract the data for each store, and use UNION ALL to concatenate the results into a single table.\n+ For each SELECT statement, we use a WHERE clause to exclude rows where the price is null for the given store.\n+ We use the AS keyword to alias the column names as required by the output format.\n+ Finally, we use ORDER BY to sort the rows by product_id and store.\n\n# Code\n```\nSELECT product_id, \'store1\' AS store, store1 AS price\nFROM Products\nWHERE store1 IS NOT NULL\nUNION ALL\nSELECT product_id, \'store2\' AS store, store2 AS price\nFROM Products\nWHERE store2 IS NOT NULL\nUNION ALL\nSELECT product_id, \'store3\' AS store, store3 AS price\nFROM Products\nWHERE store3 IS NOT NULL\nORDER BY product_id, store;\n```\n\n# SOLUTION-2: \nUNION\n\n# EXPLANATION\n+ This query is almost identical to the previous one, but it uses the UNION operator instead of UNION ALL. \n+ The difference is that UNION **removes duplicates** from the result set, whereas UNION ALL keeps all rows, even if they are duplicates. \n+ Since we know that each product can only have one price per store, there are no duplicate rows in the output, so using UNION instead of UNION ALL is safe and can **potentially improve performance** by reducing the amount of data that needs to be processed.\n\nI hope this alternative solution helps!\n\n# Code\n```\nSELECT product_id, \'store1\' AS store, store1 AS price\nFROM Products\nWHERE store1 IS NOT NULL\nUNION\nSELECT product_id, \'store2\' AS store, store2 AS price\nFROM Products\nWHERE store2 IS NOT NULL\nUNION\nSELECT product_id, \'store3\' AS store, store3 AS price\nFROM Products\nWHERE store3 IS NOT NULL\nORDER BY product_id, store;\n\n```\n\n# SOLUTION-3: \nFROM (NESTED QUERY)\n\n# EXPLANATION\nThis query is similar to the previous examples, but instead of using separate SELECT statements for each store, it uses a subquery with UNION ALL to combine the results.\n\nThe subquery selects the product_id, store name, and price for each store where the price is not NULL. The outer query simply selects the three columns from the subquery and orders the result by product_id and store.\n\nThis approach can be useful if there are many columns to select from, or if there are many stores to check for each product. It can also be more efficient than using multiple SELECT statements if the database system can optimize the subquery execution.\n\nOverall, there are many ways to write the SQL query for rearranging the Products table, and the best approach may depend on the specific database system and data being queried\n# Code\n```\nSELECT product_id, store, price\nFROM (\n  SELECT product_id, \'store1\' AS store, store1 AS price FROM Products WHERE store1 IS NOT NULL\n  UNION ALL\n  SELECT product_id, \'store2\' AS store, store2 AS price FROM Products WHERE store2 IS NOT NULL\n  UNION ALL\n  SELECT product_id, \'store3\' AS store, store3 AS price FROM Products WHERE store3 IS NOT NULL\n) AS tmp\nORDER BY product_id, store;\n\n```\n\n# SOLUTION-4: \nCASE\n\n# EXPLANATION\nThis query uses a CASE statement to determine which store has a price for each product. The query selects the product_id, store name, and price for each store where the price is not NULL. The WHERE clause filters out rows where all stores have NULL prices. Finally, the result is ordered by product_id and store.\n\nThis approach can be useful if there are many stores to check for each product and the number of stores is not fixed. However, it can be less efficient than the previous examples, especially if there are many rows with NULL prices, since the database system has to evaluate multiple CASE statements for each row.\n\nOverall, there are many ways to write the SQL query for rearranging the Products table, and the best approach may depend on the specific database system and data being queried.\n\n# Code\n```\nSELECT product_id, store, price\nFROM (\n  SELECT \n    product_id,\n    \'store1\' AS store,\n    CASE WHEN store1 IS NOT NULL THEN store1 END AS price\n  FROM Products\n  UNION ALL\n  SELECT \n    product_id,\n    \'store2\' AS store,\n    CASE WHEN store2 IS NOT NULL THEN store2 END AS price\n  FROM Products\n  UNION ALL\n  SELECT \n    product_id,\n    \'store3\' AS store,\n    CASE WHEN store3 IS NOT NULL THEN store3 END AS price\n  FROM Products\n) AS rearranged\nWHERE price IS NOT NULL\nORDER BY product_id, store;\n```\n\nI hope all solutions helps to build the understanding.
['MySQL']