從3列獲取DISTINCT值

Question

我有3個列的1個單一表格，說col1，col2，col3

值：

col1 with values (in individual 3 rows) as a, b, c
col2 with values (in individual 3 rows) as b, c, d
col3 with values (in individual 3 rows) as c, d, e

目標：

要獲取要在html下拉框中填充的獨特列表（即，我想要a，b，c，d，e），按字母順序排列？

Answer 1

偽代碼：

Select col1 from table
UNION
Select col2 from table
UNION
Select col3 from table