how to join table by itself

for project purposes only. please help me

20th Nov 2019, 4:18 PM
Annabhel Jane Melca
Annabhel Jane Melca - avatar
1 Answer
+ 8
Annabhel Jane Melca To join an table with itself is called as self join Self join == cross product + where condition For example you have an example with table as study and study table contains S_id and C_id and table contains Study table:- --------------------------- S_id | C_id S1 | C1 S2 | C2 S1 | C2 ---------------------------- Now let this table as T1 and make an copy of this table and name as T2 and then do cross product which will give T1 T2 S1C1 S1C1 S1C1 S2C2 S1C1 S1C2 S2C2 S1C1 S2C2 S2C2 S2C2 S1C2 S1C2 S1C1 S1C2 S2C2 S1C2 S1C2 Now select those student who is enrolled in at least two courses The query will be written as:- Select T1.S_id from Study as T1, Study as T2 Where T1.S_id = T2. S_id and T1.C_id <> T2.C_id Where this sign <> is for not equal to This above query is for self joining table. Hopefully it will clear your some doubts😉
20th Nov 2019, 4:49 PM