[SQL-378] - How to join only one row in joined table in SQL

sql-queries-11/single-row-from-join/create-table.sql

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+CREATE TABLE IF NOT EXISTS countries
+(
+    country_id integer NOT NULL,
+    country_name text,
+    CONSTRAINT countries_pkey PRIMARY KEY (country_id)
+);
+
+CREATE TABLE IF NOT EXISTS cities
+(
+    city_id integer NOT NULL,
+    country_id integer,
+    city_name text,
+    population bigint,
+    CONSTRAINT cities_pkey PRIMARY KEY (city_id),
+    CONSTRAINT country_id FOREIGN KEY (country_id)
+        REFERENCES countries (country_id)
+);
+
+INSERT INTO countries (country_id, country_name)
+    VALUES
+        (1, 'USA'),
+        (2, 'UK'),
+        (3, 'France'),
+        (4, 'Thailand');
+
+INSERT INTO cities (city_id, country_id, city_name, population)
+    VALUES
+	( 1, 1, 'Washington',689545),
+        ( 2,1,'New York', 8804190),
+        ( 3, 2, 'London', 12208100 ),
+        ( 4 , 2, 'Manchester', 2732854),
+        ( 5, 3, 'Paris', 2048472),
+        ( 6, 3, 'Nice', 353701),
+        ( 7, 4, 'Bangkok', 5588222),
+        ( 8, 4, 'Phuket', 77778);

sql-queries-11/single-row-from-join/single-row-from-join.sql

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+SELECT countries.country_id, country_name, city_id, city_name, population
+FROM countries
+JOIN cities ON countries.country_id = cities.country_id;
+
+SELECT DISTINCT ON(countries.country_id) 
+    countries.country_id, country_name, city_id, city_name, population
+FROM countries
+JOIN cities ON countries.country_id = cities.country_id;
+
+SELECT DISTINCT ON(countries.country_id) 
+    countries.country_id, country_name, city_id, city_name, population
+FROM countries
+JOIN cities ON countries.country_id = cities.country_id
+ORDER BY countries.country_id, population DESC;