如何根据列内容前4个字母创建索引？

Question

How can I create index on the basis of column content first 4 letter?

For example I have a table having multiple fields, one field is having name country, all Country name is in that table.

RANK    COUNTRY
--------------------------
1       Qatar
2       Luxembourg
3       Singapore
4       Brunei Darussalam
5       Kuwait
6       Norway
7       United Arab Emirates
8       Switzerland
9       United States
10      Hong Kong SAR

Now I want to do the indexing on Country field. But I want the indexing on first 4 letter of that filed data.

Answer 1

This is called a "prefix" index:

INDEX(country(4))

But I don't recommend it. There are under 300 countries in this world; you may as well index the full strings.