Python · Theory

JOIN

Theory

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JOIN

JOIN is about talking to a database from Python. Pattern is always: connect → cursor → execute → fetch or commit → close.

For JOIN: use ? placeholders (or the driver’s params). Never build SQL with + user text — that is injection. sqlite3 is in the stdlib for practice; MySQL/Mongo need a driver.

JOIN on the board — insert Asha, select her back, print the name. Same idea on MySQL. Close the connection.

Output — ('Asha', 90). JOIN on id.

Exam tip

Steps in order: connect → execute → fetch/commit → close. Mention placeholders.

Example

# JOIN
import sqlite3
con = sqlite3.connect(":memory:")
cur = con.cursor()
cur.execute("create table s(id integer, name text)")
cur.execute("create table m(sid integer, marks integer)")
cur.execute("insert into s values(1,'Asha')")
cur.execute("insert into m values(1,90)")
print(cur.execute("select s.name, m.marks from s join m on s.id=m.sid").fetchone())
con.close()

Output: ('Asha', 90). JOIN on id.

Short notes

DefJOIN uses connect + cursor + SQL/API.
RuleJOIN — parameterized queries. Never + user text into SQL.
RememberJOIN — commit after write. close when done.
TrapJOIN — open connections left hanging; passwords in source.

# JOIN import sqlite3 con = sqlite3.connect(":memory:") cur = con.cursor() cur.execute("create table s(id integer, name text)") cur.execute("create table m(sid integer, marks integer)") cur.execute("insert into s values(1,'Asha')") cur.execute("insert into m values(1,90)") print(cur.execute("select s.name, m.marks from s join m on s.id=m.sid").fetchone()) con.close()

JOIN

Example

Short notes

Questions

JOIN

Example

Short notes

Questions