Minhas consultas favoritas do PostgreSQL e por que elas são importantes

2. Concatenação de String

postgres=> SELECT 2||' times'||' 2 equals: '|| 2*2;
?column?
---------------------
2 times 2 equals: 4
(1 row)

postgres=> SELECT concat('Josh ','Otwell') AS first_name;
first_name
-------------
Josh Otwell
(1 row)

postgres=> SELECT concat('Josh',' ','Otwell') AS first_name;
first_name
-------------
Josh Otwell
(1 row)

postgres=> SELECT CONCAT('Josh',NULL,'Otwell') AS first_name;
first_name
------------
JoshOtwell
(1 row)

postgres=> SELECT 'Josh '||NULL||'Otwell' AS first_name;
first_name
------------
(1 row)

postgres=> SELECT NULL||'Josh '||'Otwell' AS first_name;
first_name
------------
(1 row)

postgres=> SELECT CONCAT(NULL,'Josh','Otwell') AS first_name;
first_name
------------
JoshOtwell
(1 row)

postgres=> SELECT concat_ws('-',333,454,1919) AS cell_num;
cell_num
--------------
333-454-1919
(1 row)

postgres=> SELECT concat_ws(' ','Josh','Otwell') AS first_name;
first_name
-------------
Josh Otwell
(1 row)

postgres=> SELECT concat_ws('-',333,NULL,1919) AS cell_num;
cell_num
----------
333-1919
(1 row)

postgres=> SELECT concat_ws(NULL,333,454,1919) AS cell_num;
cell_num
----------
(1 row)

dvdrental=> SELECT first_name||' '||last_name||'''s email address is: '||email AS name_and_email
FROM customer
LIMIT 10;
name_and_email
--------------------------------------------------------------------------
Jared Ely's email address is: [email protected]
Mary Smith's email address is: [email protected]
Patricia Johnson's email address is: [email protected]
Linda Williams's email address is: [email protected]
Barbara Jones's email address is: [email protected]
Elizabeth Brown's email address is: [email protected]
Jennifer Davis's email address is: [email protected]
Maria Miller's email address is: [email protected]
Susan Wilson's email address is: [email protected]
Margaret Moore's email address is: [email protected]
(10 rows)

3. Fornecendo a lista de valores IN com subconsultas

dvdrental=> SELECT first_name, last_name, email
FROM customer
WHERE customer_id IN (
SELECT customer_id FROM (
SELECT DISTINCT customer_id, SUM(amount)
FROM payment
WHERE extract(month from payment_date) = 4
AND extract(day from payment_date) BETWEEN 10 AND 13
GROUP BY customer_id
HAVING SUM(amount) > 30
ORDER BY SUM(amount) DESC
LIMIT 5) AS top_five);

dvdrental=> SELECT first_name, last_name, email
FROM customer
WHERE customer_id IN
(SELECT DISTINCT customer_id, SUM(amount)
FROM payment
WHERE extract(month from payment_date) = 4
AND extract(day from payment_date) BETWEEN 10 AND 13
GROUP BY customer_id
HAVING SUM(amount) > 30
ORDER BY SUM(amount) DESC
LIMIT 5);
ERROR: subquery has too many columns
LINE 3: WHERE customer_id IN (

4. gerar_series()

trial=> CREATE TABLE tbl_1(
trial(> tb_id SERIAL PRIMARY KEY,
trial(> some_day DATE,
trial(> an_amt NUMERIC(4,2));
CREATE TABLE

trial=> INSERT INTO tbl_1(some_day, an_amt)
VALUES(
generate_series('2018-04-01','2018-04-15',INTERVAL '1 day'),
generate_series(2.43, 34.20, 1.03));
INSERT 0 31

trial=> CREATE TABLE tbl_2(
tb2_id SERIAL PRIMARY KEY,
some_day2 DATE,
an_amt2 NUMERIC(4,2));
CREATE TABLE

trial=> INSERT INTO tbl_2(some_day2, an_amt2)
VALUES(
generate_series('2018-05-16','2018-05-31',INTERVAL '1 day'),
generate_series(15.43, 31., 1.03));
INSERT 0 16

5. Consultas com a função de agregação COUNT() .

dvdrental=> SELECT COUNT(*)
dvdrental-> FROM film;
count
-------
1000
(1 row)

dvdrental=> SELECT COUNT(*)
dvdrental-> FROM film
dvdrental-> WHERE rating = 'G';
count
-------
178
(1 row)

• COUNT(*) retorna o total de todas as linhas de entrada (incluindo NULLS e duplicatas).
• COUNT(alguma_expressão ) conta o número de linhas de entrada não NULL.

dvdrental=> SELECT COUNT(DISTINCT rating) FROM film;
count
-------
5
(1 row)

6. UPDATE várias linhas com uma expressão CASE.

trial=> SELECT * FROM reward_members;
rm_id | expense_amt | member_status
-------+-------------+---------------
1 | 1245.33 | gold
2 | 1300.49 | gold
3 | 900.20 | bronze
4 | 2534.44 | platinum
5 | 600.19 | bronze
6 | 1001.55 | silver
7 | 1097.99 | silver
8 | 3033.33 | platinum
(8 rows)

trial=> UPDATE reward_members
SET member_status = (
CASE member_status
WHEN 'gold' THEN 'gold_group'
WHEN 'bronze' THEN 'bronze_group'
WHEN 'platinum' THEN 'platinum_group'
WHEN 'silver' THEN 'silver_group'
END
)
WHERE member_status IN ('gold', 'bronze','platinum', 'silver');
UPDATE 8

trial=> SELECT * FROM reward_members;
rm_id | expense_amt | member_status
-------+-------------+----------------
1 | 1245.33 | gold_group
2 | 1300.49 | gold_group
3 | 900.20 | bronze_group
4 | 2534.44 | platinum_group
5 | 600.19 | bronze_group
6 | 1001.55 | silver_group
7 | 1097.99 | silver_group
8 | 3033.33 | platinum_group
(8 rows)

7. COPIAR e \copiar

trial=# SELECT expense_amt, member_status
trial-# FROM reward_members
trial-# WHERE member_status = 'gold_group';
expense_amt | member_status
-------------+---------------
1245.33 | gold_group
1300.49 | gold_group
(2 rows)

trial=# COPY (SELECT expense_amt, member_status
FROM reward_members
WHERE member_status = 'gold_group')
TO '/home/linux_user_here/awards_to_honor.csv'
DELIMITER ','
CSV HEADER;
COPY 2

$ cat awards_to_honor.csv
expense_amt,member_status
1245.33,gold_group
1300.49,gold_group

$ sudo chown postgres awards_to_honor.csv

trial=# COPY (SELECT expense_amt, member_status
FROM reward_members
WHERE member_status = 'gold_group')
TO '/tmp/awards_to_honor.csv'
DELIMITER ','
CSV HEADER;
COPY 2

trial=# SET role log_user; -- changing from postgres user to log_user
SET

trial=> COPY (SELECT expense_amt, member_status
FROM reward_members
WHERE member_status = 'gold_group')
TO '/tmp/awards_to_honor2.csv'
DELIMITER ','
CSV HEADER;
ERROR: must be superuser to COPY to or from a file
HINT: Anyone can COPY to stdout or from stdin. psql's \copy command also works for anyone.

trial=> \copy (SELECT expense_amt, member_status
FROM reward_members
WHERE member_status = 'silver_group')
TO '/home/linux_user_here/more_awards.csv'
DELIMITER ','
CSV HEADER;
COPY 2

$ cat more_awards.csv
expense_amt,member_status
1001.55,silver_group
1097.99,silver_group

trial=> \copy (SELECT expense_amt, member_status
FROM reward_members
WHERE member_status = 'silver_group')
TO '/tmp/more_awards.csv'
DELIMITER ','
CSV HEADER;
COPY 2

trial=> \! cat /tmp/more_awards.csv
expense_amt,member_status
1001.55,silver_group
1097.99,silver_group

8. meta-comando \help do psql

trial=> \help CREATE INDEX
Command: CREATE INDEX
Description: define a new index
Syntax:
CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] name ] ON table_name [ USING method ]
( { column_name | ( expression ) } [ COLLATE collation ] [ opclass ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )
[ WITH ( storage_parameter = value [, ... ] ) ]
[ TABLESPACE tablespace_name ]
[ WHERE predicate ]

Conclusão