SQL LEFT JOIN 关键字: LEFT JOIN 关键字会从左表 (table_name1) 那里返回所有的行，即使在右表 (table_name2) 中没有匹配的行

desc programming_lang;

desc cnt;

 

SELECT * from programming_lang;

SELECT * from cnt;

SELECT
a.id as lang_id,
a.name as name,
b.cnt as cnt
FROM programming_lang a
LEFT JOIN cnt b on a.id=b.lang_id;

SELECT
a.id as lang_id,
a.name as name,
b.cnt as cnt
FROM programming_lang a
right JOIN cnt b on a.id=b.lang_id;