补充多表查询语法：
SELECT column_name(s)
FROM table1

LEFT JOIN table2

ON table1.column_name=table2.column_name;

举例：
SELECT a.id,b.name,a.xxx,c.yyy FROM tableA AS a
LEFT JOIN tableB AS b ON a.id=b.id
LEFT JOIN tableC AS c ON c.idn = a.idn
WHERE b.name="张三";