SQL语法学习（5）——跨表查询JOIN

子查询

data库下现有data1和data2两张表，都包含GPS定位信息GPSID，data2表中有river信息，现在要找出data1中长江范围内的表格信息：

SELECT * FROM data.data1
WHERE GPSID=(
SELECT GPSID FROM data.data2
WHERE river="长江")

JOIN ：类似于Excel的Vlookup

我们进一步将需求改为：合并两表信息，则需要用到JOIN

SELECT * FROM data.dataanalyst AS d
JOIN data.company AS c ON dataanalyst.companyID =company.companyID

#这里用AS将表格表明信息简化

SELECT * FROM data.dataanalyst AS d
LEFT JOIN (SELECT * FROM data.company 
WHERE companySize ='150-500人')AS c 
ON t.companyID =d.companyID
WHERE t.companyID IS NULL