clickhouse Columns are from different tables - sql

Question

i want to calculate the contain relationship ,Columns a.fistname and b.fullname are from different tables a and b,for example：
CREATE TABLE users (uid Int16, firstname String) ENGINE=Memory;
INSERT INTO users VALUES (1231, 'john');
INSERT INTO users VALUES (6666, 'ksenia');
INSERT INTO users VALUES (8888, 'alice');
CREATE TABLE users_money (fullname String, money Int16) ENGINE=Memory;
INSERT INTO users_money VALUES ('johnblack', 33);
INSERT INTO users_money VALUES ('jonhwhite', 48);
INSERT INTO users_money VALUES ('aliceblack', 50);
SELECT a.firstname,b.fullname,b.money FROM users a LEFT JOIN users_money b on startswith(b.fullname,a.firstname)==1;
i hope this startswith can work,but i got error
" Unsupported JOIN ON conditions. Unexpected 'startswith(fullname, firstname)': While processing startswith(fullname, firstname). (INVALID_JOIN_ON_EXPRESSION)"
you can try: https://fiddle.clickhouse.com/
the result i want:
john johnblack 33
john Jonhwhite 48
ksenia 0
alice aliceblack 50

Answer 1

JOIN conditions require an equality operator:
https://clickhouse.com/docs/en/sql-reference/statements/select/join/#on-section-conditions
For example you can split the full name and reuse the first element in the JOIN condition
CREATE TABLE users (uid Int16, firstname String) ENGINE=Memory;
INSERT INTO users VALUES (1231, 'John');
INSERT INTO users VALUES (6666, 'Ksenia');
INSERT INTO users VALUES (8888, 'Alice');
CREATE TABLE users_money (uid Int16,fullname String, money Int16) ENGINE=Memory;
INSERT INTO users_money VALUES (1231,'John Black', 33);
INSERT INTO users_money VALUES (1231,'Jonh White', 48);
INSERT INTO users_money VALUES (8888,'Alice Black', 50);
SELECT a.firstname, splitByWhitespace(b.fullname)[1] as splitFN,b.money FROM users a LEFT JOIN users_money b on a.firstname == splitFN
Output:
John John 33
Ksenia 0
Alice Alice 50