MySQL Add foreign Key Association

SELECT * from stu st,course co,score sc where st.sid = sc.sid and sc.cid = co.cid

If we are going to give SID a constraint, that is, the SID in the two tables should correspond, so we're going to add the foreign key

Score Association Stu:

At this point, change the value of the SID in score, because the SID here must correspond to the SID in Stu, Stu does not have sid= 1008, so this time change the SID in score = 1008, when saved will prompt an error

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MySQL Add foreign Key Association