PostgreSQL – Función FIRST_VALUE

PRIMER_VALOR()

Syntax:
FIRST_VALUE ( expression )  
OVER ( 
    [PARTITION BY partition_expression, ... ]
    ORDER BY sort_expression [ASC | DESC], ...
)

Analicemos la sintaxis anterior:

• expresión
• La cláusula PARTITION BY se usa para dividir filas en un conjunto de resultados.
• ORDENAR POR
• cláusula_rango_filas

Primero, cree dos tablas Mammals Animal_groups

CREATE TABLE Animal_groups (
    animal_id serial PRIMARY KEY,
    animal_name VARCHAR (255) NOT NULL
);

CREATE TABLE Mammals (
    mammal_id serial PRIMARY KEY,
    mammal_name VARCHAR (255) NOT NULL,
    lifespan DECIMAL (11, 2),
    animal_id INT NOT NULL,
    FOREIGN KEY (animal_id) REFERENCES Animal_groups (animal_id)
);

Ahora inserte algunos datos en la tabla:

INSERT INTO Animal_groups (animal_name)
VALUES
    ('Terrestrial'),
    ('Aquatic'),
    ('Winged');

INSERT INTO Mammals(mammal_name, animal_id, lifespan)
VALUES
    ('Cow', 1, 10),
    ('Dog', 1, 7),
    ('Ox', 1, 13),
    ('Wolf', 1, 11),
    ('Blue Whale', 2, 80),
    ('Dolphin', 2, 5),
    ('Sea Horse', 2, 3),
    ('Octopus', 2, 8),
    ('Bat', 3, 4),
    ('Flying Squirrels', 3, 1),
    ('Petaurus', 3, 2);

PRIMER_VALOR()

SELECT 
    mammal_id,
    mammal_name,
    mammal_id,
    lifespan,
    FIRST_VALUE(mammal_name) 
    OVER(
        ORDER BY lifespan
    ) lowest_lifespan
FROM 
    Mammals;

Producción:

Ejemplo 2:

PRIMER_VALOR()

SELECT 
    mammal_id,
    mammal_name,
    mammal_id,
    lifespan,
    FIRST_VALUE(mammal_name) 
    OVER(
    PARTITION BY animal_id
        ORDER BY lifespan
        RANGE BETWEEN 
            UNBOUNDED PRECEDING AND 
            UNBOUNDED FOLLOWING
    ) lowest_lifespan
FROM 
    Mammals;

Producción: