SQL仅在数据存在时才从其他表中选择colmn

Question

I have two tables and I'm selecting data from the first table. If the condition is meet(Alias type = 2) and the data in second table for the id from the first table exist the I want to select the column from the second table. Here is data to explain more:

Table 1

id | Name         | Location  
---+--------------+---------  
34 |John Smith    |NewYork  
36 |Mike Smith    |London  
45 |Bob Smith     |Los Angeles  

Table 2

id | Alias             | Alias type     
---+-------------------+-------  
36 |Warren Johnson     |1  
36 |William Williams   |2  

Wanted results if alias type = 2

id |Name               | Location   
---+-------------------+---------  
34 |John Smith         |NewYork  
36 |William Williams   |London  
45 |Bob Smith          |Los Angeles  

Can you help me write query to get the wanted results? Tell me if I need to explain more. I'm using a SQL Server database.

Answer 1

You can use a left outer join onto Table2, and then COALESCE the Alias and Name results like this:

SELECT Table1.ID, COALESCE(Table2.Alias, Table1.Name) AS Name, 
    Table1.Location
FROM Table1
LEFT OUTER JOIN Table2 ON Table1.ID = Table2.ID AND Table2.AliasType = 2

COALESCE works by finding the first non-null value in the supplied values.

Answer 2

Try something like this -- you just need to use CASE :

SELECT T1.Id, 
   CASE WHEN T2.Alias IS NOT NULL THEN T2.Alias ELSE T1.Name END as Name,
   t1.Location
FROM Table1 T1
   LEFT JOIN Table2 T2 ON T1.Id = T2.Id AND T2.AliasType = 2

Here is the SQL Fiddle .

Good luck.

Answer 3

The COALESCE function will let you have a fall-back expression so that the name from table 2 is used if it exists, and the name from table 1 is used otherwise. The LEFT outer join only joins table 2 when the rows exist, and t2.name is null if no record was joined.

SELECT t1.id, COALESCE(t2.name, t1.name), t1.location FROM [Table 1] as t1 LEFT JOIN [Table 2] as t2 on t1.id=t2.id and t2.[alias type]=2