Updating one SQL table based on data in another table

问题

I am running Microsoft SQL Server 2008 R2, and pulling information from two tables to create one new table.

Table A has leads with a unique lead number and other information.
Table B has sales with a unique sales number, and the lead number associated with it.
Data from both tables are pulled into temp tables in SQL Server so I can change and update whatever I need, and the output of this will go into a new table.

One lead from Table A can have multiple sales associated with it in table B.

I want to update the Number of Sales column in Table A (Leads) based on how many times that lead number appears in Table B (sales). So if Table B (sales) has a lead number tied to seven (7) sales, the Number of Sales column in Table A (leads) will be updated to 7.

I have tried a few variations using the COUNT function but with no success. Any help would be appreciated.

回答1:

This should work for you assuming the field name is leadNo:

update tablea 
set sales = (select count(*) 
             from tableb 
             where tableb.leadNo = tablea.leadNo)

SQL Fiddle Demo

来源：https://stackoverflow.com/questions/15621826/updating-one-sql-table-based-on-data-in-another-table