Case when based on first few characters - sql

Question

I have Table A that looks like this:
Name Phone
John 1111231234
Joe 1111231235
Jack 2221231234
Jenny 2224321234
Jody 3323214211
and Table B that looks like this:
AreaCode
111
111
222
222
How do I return a result that looks like this? I essentially want to return AreaCode if the first 3 numbers/characters from the column 'Phone' exist in the column 'AreaCode' in table B...
Name Phone AreaCode
John 1111231234 111
Joe 1111231235 111
Jack 2221231234 222
Jenny 2224321234 222
Jody 3323214211 null

Answer 1

Use a left join to table b, joining where the phone starts with the areacode:
select
name,
phone,
areacode
from tableA
left join tableB on phone like concat(areacode, '%')

Answer 2

I used distinct to avoid repeated areacodes that will bring you duplicated results.
If you store areacode an phone as numeric you can omit the cast
select a.*, b.AreaCode
from TableA a left join (select distinct areacode from tableb) b
on left(cast(a.Phone as varchar(20)),3)=cast(b.AreaCode as varchar(20))

Answer 3

In teradata, you may need something like this
select
name,
phone,
areacode
from tableA
left join tableB on SUBSTRING(phone FROM 1 FOR 3) = areacode

Answer 4

As some values in column phone exceeds the max allowed limit for INTEGER i.e. 2147483647, so i assume that BIGINT is the datatype. In this case below is the query that will return your desired result.
SELECT DISTINCT t1.Name,
t1.Phone,
t2.areacode
FROM t1
LEFT JOIN t2 ON substring(t1.Phone
FROM 11
FOR 3) = t2.areacode
ORDER BY 3 DESC;
substring impliticitly cast bigint to 20 space variable character left justified as bigint requires 20 character, so the starting point for substring is 11. Also teradata implicitly compare character with INT, so no need to cast areacode
Other option is to use trim before substring to remove leading spaces as below.
SELECT DISTINCT t1.Name,
t1.Phone,
t2.areacode
FROM t1
LEFT JOIN t2 ON substring(trim(t1.Phone)
FROM 1
FOR 3) = t2.areacode
ORDER BY 3 DESC;
DISTINCT is used to avoid duplicates, as area code has duplicate values, so joining it to the phone record table will create duplicate rows.
Result:
Name Phone areacode
----------------------------------
Jack 2,221,231,234 222
Jenny 2,224,321,234 222
Joe 1,111,231,235 111
John 1,111,231,234 111
Jody 3,323,214,211 ?
P.S. As substring and trim functions are same in Teradata and MySQL, you can check the demo here
Hope this will help :-)