你的 SQL 语法不正确。试试这个:

  cursor.execute("
    SELECT t1.ID, t1.post_date, t2.meta_value AS first_name, t3.meta_value AS last_name
    FROM test_posts t1 
    LEFT JOIN test_postmeta t2 ON t1.ID = t2.post_id 
    LEFT JOIN test_postmeta t3  ON t1.ID = t3.post_id 
    WHERE t3.meta_key = '_billing_last_name' and t2.meta_key = '_billing_first_name'
    GROUP BY t1.ID 
    ORDER BY t1.post_date DESC LIMIT 20")

也许值得一读 SQL语言 Joins 和 WHERE 声明。

import pandas as pd

df = pd.read_csv("test.csv")
df['count'] = pd.to_numeric(df['count'])
#df['count'] = df.groupby(['year'])['count'].sum()

total = df.groupby(['year'])['count'].sum()

print(total)

产量:

year
2001    46
2004    30
2005    21