SELECT Lists

Select One Column

A SELECT list can copy one column.

highlighted = computed this step

Select one column

A one-column SELECT list copies that column into the output.

SELECT item\text{SELECT item}

Read one output column

The output has 1 column and 4 rows.

output columns=1\text{output columns}=1

SELECT-list examples are tiny finite-table transforms; DISTINCT, aggregation, type coercion, optimizer behavior, and database-product claims are out of scope.

Select one column: base rowsitemqtybonusstatuspen31openmug20opencup21closedtagNULL1open SELECT list itemspositionkindsourceoutput0columnitemitem SELECT output rowsitempenmugcuptag SELECT factsfactvalueinputRowCount4outputRowCount4outputColumnCount1duplicateOutputRows0rowOrdersource_orderdistinctAppliedno

Rows are not collapsed

Selecting one column still visits every input row.

same row count\text{same row count}

Summary

A SELECT list changes shape, not source order.

shape change\text{shape change}