Conditional JOIN between tables? - sql

Question

I have two table and I should write a select query which join this two table but I do not know what is conditional join between this two tables?
Can some body say what is?
TABLE ParameterRegistration
(
RegistrationTime DATETIME,
PatiNo VARCHAR(12),
Source VARCHAR(64),
Code VARCHAR(64),
NameOfCodingSystem VARCHAR(64) NULL,
Name VARCHAR(64),
ValueType CHAR(2) NULL,
NumericValue INT NULL,
StringValue VARCHAR(64) NULL,
TextValue TEXT NULL,
Unit VARCHAR(64) NULL,
UnitCode VARCHAR(64) NULL,
UnitCodingSystem VARCHAR(64) NULL,
Remark VARCHAR(255) NULL,
CreateDate DATETIME,
CreateUserId T_USER_ID
)
and
TABLE External
(
ModDate DATETIME,
ModUserId VARCHAR(12),
UbMem VARCHAR(64),
Code VARCHAR(64),
Name VARCHAR(64),
Service VARCHAR(64),
NameOfCodingSystem VARCHAR(64) NULL,
)

Answer 1

You can only properly join those two tables if they have a relation of some kind.
Check THIS ARTICLE for some info.
Assuming that in your case, the tables are related with the columns Code, NameOfCodingSystem and Name you can make a join like this:
select p.*, e.* from ParameterRegistration p
inner join External e on p.Code = e.Code and
p.NameOfCodingSystem = e.NameOfCodingSystem and
p.Name = e.Name

Answer 2

When joining tables you have to join on fields that are related. Since you did not provide a lot of information it looks like you have 3 fields that possibly could be joined on.
NameOfCodingSystem VARCHAR(64)
Name VARCHAR(64)
Code VARCHAR(64)
So you could technically write your query one of these ways:
SELECT *
FROM ParameterRegistration P
INNER JOIN External E -- or LEFT JOIN, etc
ON P.NameOfCodingSystem = E.NameOfCodingSystem
OR
SELECT *
FROM ParameterRegistration P
INNER JOIN External E -- or LEFT JOIN, etc
ON P.Name = E.Name
OR
SELECT *
FROM ParameterRegistration P
INNER JOIN External E -- or LEFT JOIN, etc
ON P.Code = E.Code
OR you can join on all of the fields at the same time
SELECT *
FROM ParameterRegistration P
INNER JOIN External E -- or LEFT JOIN, etc
ON P.NameOfCodingSystem = E.NameOfCodingSystem AND
P.Name = E.Name AND
P.Code = E.Code
My suggestion would be to study up on JOINs. Here are some resources but there are plenty on the internet:
A Visual Explanation of Joins
SQL Joins
Joins