-- fake table data
CREATE TABLE sellers SELECT 1 sno, 0 flag UNION SELECT 2, 0 UNION SELECT 3, 0;
CREATE TABLE products SELECT 1 sellerid UNION ALL SELECT 1 UNION ALL SELECT 2;
SET sql_mode := '';

-- initial query
SELECT e.*, count(*) AS `count` 
FROM sellers AS e  
left join products AS r ON e.sno = r.sellerid 
where NOT e.flag='1' 
GROUP BY e.sno

snoflagcount102201301号

-- counting joined values, not rows (NULLs are ignored)
SELECT e.*, count(r.sellerid) AS `count` 
FROM sellers AS e  
left join products AS r ON e.sno = r.sellerid 
where NOT e.flag='1' 
GROUP BY e.sno
-- further filtering by count>0 needed

snoflagcount102201300号

-- join only rows which have matched rows in second table
SELECT e.*, count(*) AS `count` 
FROM sellers AS e  
inner join products AS r ON e.sno = r.sellerid 
where NOT e.flag='1' 
GROUP BY e.sno
-- no further filtering needed

snoflagcount102201号
db<>在这里摆弄