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| <h1 class="title topictitle1" id="ariaid-title1">DISTINCT Operator</h1> |
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| <p class="p"> |
| The <code class="ph codeph">DISTINCT</code> operator in a <code class="ph codeph">SELECT</code> statement filters the |
| result set to remove duplicates. |
| </p> |
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| <pre class="pre codeblock"><code>-- Returns the unique values from one column. |
| -- NULL is included in the set of values if any rows have a NULL in this column. |
| SELECT DISTINCT c_birth_country FROM customer; |
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| -- Returns the unique combinations of values from multiple columns. |
| SELECT DISTINCT c_salutation, c_last_name FROM customer;</code></pre> |
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| <p class="p"> |
| You can use <code class="ph codeph">DISTINCT</code> in combination with an aggregation function, |
| typically <code class="ph codeph">COUNT()</code>, to find how many different values a column contains. |
| </p> |
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| <pre class="pre codeblock"><code>-- Counts the unique values from one column. |
| -- NULL is not included as a distinct value in the count. |
| SELECT COUNT(DISTINCT c_birth_country) FROM customer; |
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| -- Counts the unique combinations of values from multiple columns. |
| SELECT COUNT(DISTINCT c_salutation, c_last_name) FROM customer;</code></pre> |
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| <p class="p"> |
| <strong class="ph b">Zero-length strings:</strong> For purposes of clauses such as <code class="ph codeph">DISTINCT</code> |
| and <code class="ph codeph">GROUP BY</code>, Impala considers zero-length strings |
| (<code class="ph codeph">""</code>), <code class="ph codeph">NULL</code>, and space to all be different values. |
| </p> |
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| <div class="note note"><span class="notetitle">Note:</span> |
| <p class="p"> |
| In contrast with some database systems that always return <code class="ph codeph">DISTINCT</code> |
| values in sorted order, Impala does not do any ordering of <code class="ph codeph">DISTINCT</code> |
| values. Always include an <code class="ph codeph">ORDER BY</code> clause if you need the values in |
| alphabetical or numeric sorted order. |
| </p> |
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| <div class="parentlink"><strong>Parent topic:</strong> <a class="link" href="../topics/impala_select.html">SELECT Statement</a></div> |
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