Python – 5-2. pandas(DataFrame) like SQL

import pandas as pd
df_emp = pd.read_csv('emp.csv', encoding='utf-8')
df_dept = pd.read_csv('dept.csv', encoding='utf-8')

集計(dept別でsalが最大のempno)

SQL
WITH grp AS (
SELECT deptno, MAX(sal) as maxsal
  FROM emp GROUP BY deptno
)
SELECT e.deptno, empno, sal
 FROM  emp e INNER JOIN grp g
       ON (e.deptno = g.deptno
       AND e.sal = g.maxsal)
ORDER BY e.deptno, empno;
pandas(deptでグループ化して最大のsalの金額でjoin)

※同金額が2件ある場合は2件とも取得される

pd.merge(df_emp, df_emp.groupby('dept')[['sal']].agg(max).reset_index() \
        ,how='inner', right_on='dept', left_on='dept') \
 .query('sal_x == sal_y') \
 .sort_values('dept')
 
==>     empno ename        job  manager    hiredate  sal_x  comm  dept  sal_y
==> 12   7839  藤原太郎  PRESIDENT      NaN  1981/11/17   5000   NaN  10.0   5000
==> 2    7788  中村健一    ANALYST   7566.0  1987/04/19   3000   NaN  20.0   3000
==> 4    7902  松本亮太    ANALYST   7566.0  1981/12/03   3000   NaN  20.0   3000
==> 8    7698   伊藤誠    MANAGER   7839.0  1981/05/01   2850   NaN  30.0   2850
pandas(deptでグループ化しsalのランク1位のみを取得)

※同金額が2件ある場合、そのうち1件しか取得されない
※deptでjoinしないのでdeptがnullの行も抽出される

pd.merge(df_emp.reset_index(), df_emp.groupby('dept')[['sal']].rank(ascending=False, method='first') \
        .reset_index(), how='inner', right_on='index', left_on='index') \
 .query('sal_y == 1.0') \
 .sort_values('dept')
 
==>    index  empno  ename        job   manager    hiredate   sal_x  comm  dept   sal_y
==> 8      8   7839  藤原太郎  PRESIDENT     NaN  1981/11/17   5000   NaN  10.0   1.0
==> 7      7   7788  中村健一   ANALYST   7566.0  1987/04/19   3000   NaN  20.0   1.0
==> 5      5   7698   伊藤誠    MANAGER   7839.0  1981/05/01   2850   NaN  30.0   1.0
pandas(先にdept,sal毎にソートし、dept毎の最上位1件だけを取得)
df_emp.sort_values(by=['dept','sal'], ascending=[True,False]).groupby('dept').head(1)
 
==>     empno ename        job  manager    hiredate   sal  comm  dept
==> 8    7839  藤原太郎  PRESIDENT      NaN  1981/11/17  5000   NaN  10.0
==> 7    7788  中村健一    ANALYST   7566.0  1987/04/19  3000   NaN  20.0
==> 5    7698   伊藤誠    MANAGER    7839.0  1981/05/01  2850   NaN  30.0
==> 14   8001  小林政男   SALESMAN   7698.0  1985/03/17  1300   NaN   NaN

集計(dept別でsalが最大の行名(行index))

SQL(rowidを取得)
WITH grp AS (
SELECT dept, MAX(sal) as maxsal
  FROM emp GROUP BY dept
)
SELECT rowid
 FROM  emp e INNER JOIN grp g
       ON (e.deptno = g.deptno
       AND e.sal = g.maxsal)
ORDER BY e.deptno, empno;
pandas(dept別のindex)
df_emp.groupby('dept')['sal'].idxmax()
==> dept
==> 10.0    8
==> 20.0    7
==> 30.0    5
==> Name: sal, dtype: int64
 
※複数件ある場合に、複数のindexを取得したい場合は、deptでgroupbyし、そのmax()と一致するdept,salを持ったindexを取得する
↓
pd.merge(
    df_emp[['dept','sal']].reset_index(),
    df_emp.groupby('dept')['sal'].max().reset_index(),
    how='inner', on=['dept','sal']) \
    .loc[:,['dept','index']].sort_values('dept').reset_index(drop=True)
==>    dept  index
==> 0  10.0      8
==> 1  20.0      7
==> 2  20.0     12
==> 3  30.0      5

縦結合(UNION)

SQL
SELECT * FROM emp WHERE dept = 10
UNION
SELECT * FROM emp WHERE dept = 20;
pandas
pd.concat([df_emp[df_emp['dept']==10], df_emp[df_emp['dept']==20]])
もしくは、
pd.concat([df_emp.query('dept==10'), df_emp.query('dept==20')])
※concatの引数はiterableなオブジェクト(リストやタプル)で指定する
 
==>     empno ename        job  manager    hiredate   sal  comm  dept
==> 6    7782  山本尚人    MANAGER   7839.0  1981/06/09  2450   NaN  10.0
==> 8    7839  藤原太郎  PRESIDENT      NaN  1981/11/17  5000   NaN  10.0
==> 13   7934  木村大輝      CLERK   7782.0  1982/01/23  1300   NaN  10.0
==> 0    7369  佐藤一郎      CLERK   7902.0  1980/12/17   800   NaN  20.0
==> 3    7566  田中義男    MANAGER   7839.0  1981/04/02  2975   NaN  20.0
==> 7    7788  中村健一    ANALYST   7566.0  1987/04/19  3000   NaN  20.0
==> 10   7876  吉田翔太      CLERK   7788.0  1987/05/23  1100   NaN  20.0
==> 12   7902  松本亮太    ANALYST   7566.0  1981/12/03  3000   NaN  20.0

厳密にUNIONの動作(重複行は削除)に合わせたいときは

pd.concat([df_emp.query('dept==10'), df_emp.query('dept==20')]).drop_duplicates()

とする(この例では重複は発生しないので変わりないが)

Posted by tfurukaw