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| <section id="sql"> |
| <h1>SQL<a class="headerlink" href="#sql" title="Link to this heading">#</a></h1> |
| <p>DataFusion also offers a SQL API, read the full reference <a class="reference external" href="https://arrow.apache.org/datafusion/user-guide/sql/index.html">here</a></p> |
| <div class="cell docutils container"> |
| <div class="cell_input docutils container"> |
| <div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">datafusion</span> |
| <span class="kn">from</span><span class="w"> </span><span class="nn">datafusion</span><span class="w"> </span><span class="kn">import</span> <span class="n">DataFrame</span><span class="p">,</span> <span class="n">SessionContext</span> |
| |
| <span class="c1"># create a context</span> |
| <span class="n">ctx</span> <span class="o">=</span> <span class="n">datafusion</span><span class="o">.</span><span class="n">SessionContext</span><span class="p">()</span> |
| |
| <span class="c1"># register a CSV</span> |
| <span class="n">ctx</span><span class="o">.</span><span class="n">register_csv</span><span class="p">(</span><span class="s2">"pokemon"</span><span class="p">,</span> <span class="s2">"pokemon.csv"</span><span class="p">)</span> |
| |
| <span class="c1"># create a new statement via SQL</span> |
| <span class="n">df</span> <span class="o">=</span> <span class="n">ctx</span><span class="o">.</span><span class="n">sql</span><span class="p">(</span><span class="s1">'SELECT "Attack"+"Defense", "Attack"-"Defense" FROM pokemon'</span><span class="p">)</span> |
| |
| <span class="c1"># collect and convert to pandas DataFrame</span> |
| <span class="n">df</span><span class="o">.</span><span class="n">to_pandas</span><span class="p">()</span> |
| </pre></div> |
| </div> |
| </div> |
| <div class="cell_output docutils container"> |
| <div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span> pokemon.Attack + pokemon.Defense pokemon.Attack - pokemon.Defense |
| 0 98 0 |
| 1 125 -1 |
| 2 165 -1 |
| 3 223 -23 |
| 4 95 9 |
| .. ... ... |
| 158 190 10 |
| 159 109 19 |
| 160 149 19 |
| 161 229 39 |
| 162 200 20 |
| |
| [163 rows x 2 columns] |
| </pre></div> |
| </div> |
| </div> |
| </div> |
| <section id="parameterized-queries"> |
| <h2>Parameterized queries<a class="headerlink" href="#parameterized-queries" title="Link to this heading">#</a></h2> |
| <p>In DataFusion-Python 51.0.0 we introduced the ability to pass parameters |
| in a SQL query. These are similar in concept to |
| <a class="reference external" href="https://datafusion.apache.org/user-guide/sql/prepared_statements.html">prepared statements</a>, |
| but allow passing named parameters into a SQL query. Consider this simple |
| example.</p> |
| <div class="cell docutils container"> |
| <div class="cell_input docutils container"> |
| <div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">show_attacks</span><span class="p">(</span><span class="n">ctx</span><span class="p">:</span> <span class="n">SessionContext</span><span class="p">,</span> <span class="n">threshold</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span> |
| <span class="n">ctx</span><span class="o">.</span><span class="n">sql</span><span class="p">(</span> |
| <span class="s1">'SELECT "Name", "Attack" FROM pokemon WHERE "Attack" > $val'</span><span class="p">,</span> <span class="n">val</span><span class="o">=</span><span class="n">threshold</span> |
| <span class="p">)</span><span class="o">.</span><span class="n">show</span><span class="p">(</span><span class="n">num</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span> |
| <span class="n">show_attacks</span><span class="p">(</span><span class="n">ctx</span><span class="p">,</span> <span class="mi">75</span><span class="p">)</span> |
| </pre></div> |
| </div> |
| </div> |
| <div class="cell_output docutils container"> |
| <div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>DataFrame() |
| +---------------------------+--------+ |
| | Name | Attack | |
| +---------------------------+--------+ |
| | Venusaur | 82 | |
| | VenusaurMega Venusaur | 100 | |
| | Charizard | 84 | |
| | CharizardMega Charizard X | 130 | |
| | CharizardMega Charizard Y | 104 | |
| +---------------------------+--------+ |
| </pre></div> |
| </div> |
| </div> |
| </div> |
| <p>When passing parameters like the example above we convert the Python objects |
| into their string representation. We also have special case handling |
| for <a class="reference internal" href="../autoapi/datafusion/dataframe/index.html#datafusion.dataframe.DataFrame" title="datafusion.dataframe.DataFrame"><code class="xref py py-class docutils literal notranslate"><span class="pre">DataFrame</span></code></a> objects, since they cannot simply |
| be turned into string representations for an SQL query. In these cases we |
| will register a temporary view in the <a class="reference internal" href="../autoapi/datafusion/context/index.html#datafusion.context.SessionContext" title="datafusion.context.SessionContext"><code class="xref py py-class docutils literal notranslate"><span class="pre">SessionContext</span></code></a> |
| using a generated table name.</p> |
| <p>The formatting for passing string replacement objects is to precede the |
| variable name with a single <code class="docutils literal notranslate"><span class="pre">$</span></code>. This works for all dialects in |
| the SQL parser except <code class="docutils literal notranslate"><span class="pre">hive</span></code> and <code class="docutils literal notranslate"><span class="pre">mysql</span></code>. Since these dialects do not |
| support named placeholders, we are unable to do this type of replacement. |
| We recommend either switching to another dialect or using Python |
| f-string style replacement.</p> |
| <div class="admonition warning"> |
| <p class="admonition-title">Warning</p> |
| <p>To support DataFrame parameterized queries, your session must support |
| registration of temporary views. The default |
| <a class="reference internal" href="../autoapi/datafusion/catalog/index.html#datafusion.catalog.CatalogProvider" title="datafusion.catalog.CatalogProvider"><code class="xref py py-class docutils literal notranslate"><span class="pre">CatalogProvider</span></code></a> and |
| <a class="reference internal" href="../autoapi/datafusion/catalog/index.html#datafusion.catalog.SchemaProvider" title="datafusion.catalog.SchemaProvider"><code class="xref py py-class docutils literal notranslate"><span class="pre">SchemaProvider</span></code></a> do have this capability. |
| If you have implemented custom providers, it is important that temporary |
| views do not persist across <a class="reference internal" href="../autoapi/datafusion/context/index.html#datafusion.context.SessionContext" title="datafusion.context.SessionContext"><code class="xref py py-class docutils literal notranslate"><span class="pre">SessionContext</span></code></a> |
| or you may get unintended consequences.</p> |
| </div> |
| <p>The following example shows passing in both a <a class="reference internal" href="../autoapi/datafusion/dataframe/index.html#datafusion.dataframe.DataFrame" title="datafusion.dataframe.DataFrame"><code class="xref py py-class docutils literal notranslate"><span class="pre">DataFrame</span></code></a> |
| object as well as a Python object to be used in parameterized replacement.</p> |
| <div class="cell docutils container"> |
| <div class="cell_input docutils container"> |
| <div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">show_column</span><span class="p">(</span> |
| <span class="n">ctx</span><span class="p">:</span> <span class="n">SessionContext</span><span class="p">,</span> <span class="n">column</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span> <span class="n">df</span><span class="p">:</span> <span class="n">DataFrame</span><span class="p">,</span> <span class="n">threshold</span><span class="p">:</span> <span class="nb">int</span> |
| <span class="p">)</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span> |
| <span class="n">ctx</span><span class="o">.</span><span class="n">sql</span><span class="p">(</span> |
| <span class="s1">'SELECT "Name", $col FROM $df WHERE $col > $val'</span><span class="p">,</span> |
| <span class="n">col</span><span class="o">=</span><span class="n">column</span><span class="p">,</span> |
| <span class="n">df</span><span class="o">=</span><span class="n">df</span><span class="p">,</span> |
| <span class="n">val</span><span class="o">=</span><span class="n">threshold</span><span class="p">,</span> |
| <span class="p">)</span><span class="o">.</span><span class="n">show</span><span class="p">(</span><span class="n">num</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span> |
| <span class="n">df</span> <span class="o">=</span> <span class="n">ctx</span><span class="o">.</span><span class="n">table</span><span class="p">(</span><span class="s2">"pokemon"</span><span class="p">)</span> |
| <span class="n">show_column</span><span class="p">(</span><span class="n">ctx</span><span class="p">,</span> <span class="s1">'"Defense"'</span><span class="p">,</span> <span class="n">df</span><span class="p">,</span> <span class="mi">75</span><span class="p">)</span> |
| </pre></div> |
| </div> |
| </div> |
| <div class="cell_output docutils container"> |
| <div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>DataFrame() |
| +---------------------------+---------+ |
| | Name | Defense | |
| +---------------------------+---------+ |
| | Venusaur | 83 | |
| | VenusaurMega Venusaur | 123 | |
| | Charizard | 78 | |
| | CharizardMega Charizard X | 111 | |
| | CharizardMega Charizard Y | 78 | |
| +---------------------------+---------+ |
| </pre></div> |
| </div> |
| </div> |
| </div> |
| <p>The approach implemented for conversion of variables into a SQL query |
| relies on string conversion. This has the potential for data loss, |
| specifically for cases like floating point numbers. If you need to pass |
| variables into a parameterized query and it is important to maintain the |
| original value without conversion to a string, then you can use the |
| optional parameter <code class="docutils literal notranslate"><span class="pre">param_values</span></code> to specify these. This parameter |
| expects a dictionary mapping from the parameter name to a Python |
| object. Those objects will be cast into a |
| <a class="reference external" href="https://arrow.apache.org/docs/python/generated/pyarrow.Scalar.html">PyArrow Scalar Value</a>.</p> |
| <p>Using <code class="docutils literal notranslate"><span class="pre">param_values</span></code> will rely on the SQL dialect you have configured |
| for your session. This can be set using the <a class="reference internal" href="configuration.html#configuration"><span class="std std-ref">configuration options</span></a> |
| of your <a class="reference internal" href="../autoapi/datafusion/context/index.html#datafusion.context.SessionContext" title="datafusion.context.SessionContext"><code class="xref py py-class docutils literal notranslate"><span class="pre">SessionContext</span></code></a>. Similar to how |
| <a class="reference external" href="https://datafusion.apache.org/user-guide/sql/prepared_statements.html">prepared statements</a> |
| work, these parameters are limited to places where you would pass in a |
| scalar value, such as a comparison.</p> |
| <div class="cell docutils container"> |
| <div class="cell_input docutils container"> |
| <div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">param_attacks</span><span class="p">(</span><span class="n">ctx</span><span class="p">:</span> <span class="n">SessionContext</span><span class="p">,</span> <span class="n">threshold</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span> |
| <span class="n">ctx</span><span class="o">.</span><span class="n">sql</span><span class="p">(</span> |
| <span class="s1">'SELECT "Name", "Attack" FROM pokemon WHERE "Attack" > $val'</span><span class="p">,</span> |
| <span class="n">param_values</span><span class="o">=</span><span class="p">{</span><span class="s2">"val"</span><span class="p">:</span> <span class="n">threshold</span><span class="p">},</span> |
| <span class="p">)</span><span class="o">.</span><span class="n">show</span><span class="p">(</span><span class="n">num</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span> |
| <span class="n">param_attacks</span><span class="p">(</span><span class="n">ctx</span><span class="p">,</span> <span class="mi">75</span><span class="p">)</span> |
| </pre></div> |
| </div> |
| </div> |
| <div class="cell_output docutils container"> |
| <div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>DataFrame() |
| +---------------------------+--------+ |
| | Name | Attack | |
| +---------------------------+--------+ |
| | Venusaur | 82 | |
| | VenusaurMega Venusaur | 100 | |
| | Charizard | 84 | |
| | CharizardMega Charizard X | 130 | |
| | CharizardMega Charizard Y | 104 | |
| +---------------------------+--------+ |
| </pre></div> |
| </div> |
| </div> |
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