试图计数并加入两个mysql表

Question

I've got a mysql db with two tables:

db.names

name | Level
------------
bob    4
john   3
andy   2
dave   1

db.data

entry | user
------------
dfds    bob
hdes    bob
sers    john
iuid    dave
yyuy    john

I'm trying to do a count that will count how many entrys each user has made and also show their level. So lit looks something like this:

count(*) | user | Level
-----------------------
2         bob     4
2         john    3
1         dave    1

I've tried to use left join and distinct join, but can seem to get to grips with the logic. Help would be greatlly appreciated

Answer 1

You need to basically joins the tables using INNER JOIN since you only want to show a record that both exist on the two tables. And to count their number of instances, you need to use an aggregate function COUNT() and a GROUP BY clause.

SELECT  COUNT(*) TotalCount,
        a.Name,
        a.Level
FROM    names a
        INNER JOIN data b
            ON a.Name = b.user
GROUP   BY a.Name, a.Level

• SQLFiddle Demo

To further gain more knowledge about joins, kindly visit the link below:

• Visual Representation of SQL Joins

OUTPUT

╔════════════╦══════╦═══════╗
║ TOTALCOUNT ║ NAME ║ LEVEL ║
╠════════════╬══════╬═══════╣
║          2 ║ bob  ║     4 ║
║          2 ║ john ║     3 ║
║          1 ║ dave ║     1 ║
╚════════════╩══════╩═══════╝