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GO Classes DPP | GATE CS, DA | DBMS | SQL Queries

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2 2 votes

Consider:

SQL Statement $\mathbf{1}$

$\mathrm{SELECT\ *}$ $\mathrm{FROM\ R}$ $\mathrm{UNION}$ $\mathrm{SELECT\ *}$ $\mathrm{FROM\ R}$;


SQL Statement $\mathbf{2}$

$\mathrm{SELECT\ *}$ $\mathrm{FROM\ R}$;


What is the necessary and sufficient condition for SQL $1$ and SQL $2$ to always produce identical results for table $\mathrm{R}$?

  1. No row of $\mathrm{R}$ contains a $\mathrm{NULL}$.
     
  2. $\mathrm{R}$ contains zero rows.
     
  3. $\mathrm{R}$ contains no duplicate rows.
     
  4. $\mathrm{R}$ contains exactly one column.

1 Answer

0 0 votes

 

Therefore the two results differ whenever $\mathrm{R}$ contains duplicate tuples.

If $\mathrm{R}$ contains no duplicate rows, removing duplicates changes nothing.

Hence,

$\mathrm{SQL\ 1}=\mathrm{SQL\ 2}$ exactly when $\mathrm{R}$ has no duplicate rows.

So C is both necessary and sufficient.

Hence,

Answer : $\boxed{\mathrm{C}}$

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