Learn Relational Database by Building a Celestial bodies database - step 10

Tell us what’s happening:
I can’t figure out what it is asking for in step 10

Blockquote STEP10: Each “star” should have a foreign key that references one of the rows in galaxy

Your code so far

-- step1
CREATE DATABASE universe; 
-- step2
\c universe; 
CREATE TABLE galaxy();
CREATE TABLE star();
CREATE TABLE planet();
CREATE TABLE moon();
-- step3 & step4
ALTER TABLE galaxy ADD COLUMN id SERIAL NOT NULL;
ALTER TABLE galaxy ADD PRIMARY KEY(id); 
ALTER TABLE star ADD COLUMN id SERIAL NOT NULL;
ALTER TABLE star ADD PRIMARY KEY(id); 
ALTER TABLE planet ADD COLUMN id SERIAL NOT NULL;
ALTER TABLE planet ADD PRIMARY KEY(id); 
ALTER TABLE moon ADD COLUMN id SERIAL NOT NULL;
ALTER TABLE moon ADD PRIMARY KEY(id); 
-- step5
ALTER TABLE galaxy ADD COLUMN name VARCHAR(30);
ALTER TABLE star ADD COLUMN name VARCHAR(30);
ALTER TABLE planet ADD COLUMN name VARCHAR(30);
ALTER TABLE moon ADD COLUMN name VARCHAR(30);
-- step6 
ALTER TABLE galaxy ADD COLUMN size INT;
ALTER TABLE star ADD COLUMN size INT;
-- step7
ALTER TABLE planet ADD COLUMN size NUMERIC;
-- step8 
ALTER TABLE moon ADD COLUMN sec_name TEXT; 
-- step9
ALTER TABLE galaxy ADD COLUMN wo BOOLEAN; 
ALTER TABLE star ADD COLUMN wo BOOLEAN; 
--step10 
ALTER TABLE star ADD FOREIGN KEY(size) REFERENCES galaxy(size);
ERROR:  there is no unique constraint matching given keys for referenced table "galaxy"

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It means that the stars table should have a column which is referencing the galaxy_id from the galaxies table.

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That’s doesn’t work

ALTER TABLE star ADD COLUMN galaxy_id INT NOT NULL REFERENCES galaxy(id);

Read the requirements again. Your galaxy table’s id column should be called galaxy_id as far as I recall. (All the id columns need to fulfill the naming requirements for all the tables in fact)