SQL¶

if¶

IF(clause, yes, no)

SELECT N, 
    IF( 
        P is NULL, 
        'Root', 
        IF(
           (SELECT COUNT(*) FROM BST WHERE P = bst_o.N) = 0, 
        'Leaf',
        'Inner'
        )
    )
FROM BST AS bst_o ORDER BY N

slice¶

RIGHT(COLUMN, 3) → COLUMN[-3:]
SUBSTRING(COLUMN, -3) → COLUMN[-3:]
SUBSTRING(COLUMN, 5, 3) → COLUMN[5:5+3]

replace¶

REPLACE(COLUMN, <before>, <after>)

regex¶

SELECT COLUMN FROM TABLE WHERE COLUMN REGEXP ''

^xx start with xx
xx$ end with xx
[aeiou] a or e or i or o or u
.* whatever
<pattern> = 0 negation

join¶

SELECT CITY.NAME
FROM CITY INNER JOIN COUNTRY
ON COUNTRYCODE = CODE
WHERE CONTINENT = 'Africa'

SELECT W.id, WP.age, W.coins_needed, W.power 
FROM Wands AS W JOIN Wands_Property AS WP
ON W.code = WP.code
WHERE WP.is_evil = 0
    AND W.coins_needed = 
    (SELECT MIN(coins_needed) 
     FROM Wands AS W1 JOIN Wands_Property AS WP1 
     ON W1.code = WP1.code
     WHERE W.power = W1.power AND WP.age = WP1.age)
ORDER BY W.power DESC, WP.age DESC

JOIN = INNER JOIN

Rank¶

Give ranking number based on the value of a column

RANK() vs. DENSE_RANK()

e.g.

val	rank	dense_rank
90	1	1
90	1	1
88	3	2

others¶

DISTINCT
AVG
COUNT
ROUND(A, 2)
TRUNCATE(A, 2)

Recursive Queries¶

Quering the subtree of id = 9 of a self-referencing table with cte (common table expressions)

with recursive cte (id, name, parent_id) as (
  select     id,
             name,
             parent_id
  from       products
  where      parent_id = 19
  union all
  select     p.id,
             p.name,
             p.parent_id
  from       products p
  inner join cte
          on p.parent_id = cte.id
)
select * from cte;

https://stackoverflow.com/a/33737203/15493213

Alter the type of a referenced column¶

Assume you have a column in a table that is a foreign key referencing a column in another table, but you want to change its type, you cannot directly change the type.

drop the foreign key
alter the column type
add back the foreign key

To see your foreign key (along with other field definitions),

SHOW CREATE TABLE table_name;

Assuming it's column author_name in table authors referencing column author_name in table books

ALTER TABLE `authors` DROP FOREIGN KEY `fk_authors_books`;
ALTER TABLE `authors` MODIFY COLUMN `author_name` varchar(16) NOT NULL
ALTER TABLE `books` ADD FOREIGN KEY (`author_name`) REFERENCES `authors` (`author_name`)