SELECT Lists

Select Several Columns

A SELECT list can copy several columns.

highlighted = computed this step

Select several columns

A SELECT list can copy several columns side by side.

several selected columns\text{several selected columns}

Read several output columns

The output has 3 selected columns.

output columns=3\text{output columns}=3

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

Select several columns: base rowsitemqtybonusstatuspen31openmug20opencup21closedtagNULL1open SELECT list itemspositionkindsourceoutput0columnitemitem1columnqtyqty2columnstatusstatus SELECT output rowsitemqtystatuspen3openmug2opencup2closedtagNULLopen SELECT factsfactvalueinputRowCount4outputRowCount4outputColumnCount3duplicateOutputRows0rowOrdersource_orderdistinctAppliedno

Same source row

Each output row is built from one source row.

row by row\text{row by row}

Summary

Several selected columns make a narrower table than the base table.

narrower output\text{narrower output}