You want to combine two queries using UNION ALL but also want to add a column indicating the source table. Which query correctly achieves this?
hard📝 Application Q9 of 15
SQL - Set Operations
You want to combine two queries using UNION ALL but also want to add a column indicating the source table. Which query correctly achieves this?
ASELECT id, 'Table1' AS source FROM table1 UNION ALL SELECT id, 'Table2' AS source FROM table2;
BSELECT id FROM table1 UNION ALL SELECT id, 'Table2' FROM table2;
CSELECT id, source FROM table1 UNION ALL SELECT id, source FROM table2;
DSELECT id, 'Table1' FROM table1 UNION SELECT id, 'Table2' FROM table2;
Step-by-Step Solution
Solution:
Step 1: Add a source column in each SELECT
SELECT id, 'Table1' AS source FROM table1 UNION ALL SELECT id, 'Table2' AS source FROM table2; adds a literal string column named source in both queries.
Step 2: Use UNION ALL to combine including duplicates
SELECT id, 'Table1' AS source FROM table1 UNION ALL SELECT id, 'Table2' AS source FROM table2; uses UNION ALL correctly with matching columns.
Step 3: Check other options for errors
A assumes source column exists in tables, B mismatches columns, D uses UNION (removes duplicates).
Final Answer:
SELECT id, 'Table1' AS source FROM table1 UNION ALL SELECT id, 'Table2' AS source FROM table2; -> Option A
Quick Check:
Adding source column with UNION ALL = SELECT id, 'Table1' AS source FROM table1 UNION ALL SELECT id, 'Table2' AS source FROM table2; [OK]
Quick Trick:Add literal column in each SELECT to identify source [OK]
Common Mistakes:
MISTAKES
Mismatching column counts
Using UNION instead of UNION ALL
Assuming source column exists in tables
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