EXPLAIN

provides detailed information about the query result. One of the information that

EXPLAIN

gives us is how many rows will be affected after running the query

EXPLAIN UPDATE students SET mail = 'email@gmail.com' WHERE id > 39

Note:

EXPLAIN

works with the following statements:

SELECT

,

DELETE

,

INSERT

,

REPLACE

,

UPDATE

Selects the specified database so that after running the query we don't need to specify the database for other queries related to the specified one

USE university

For example we can list all columns from any of the table inside the selected database without specifying it. MySQL will know that we mean already the selected database

SHOW COLUMNS FROM students

Lists all the columns from students table

DESCRIBE students

Note: the query above is alternative and shortcut of

SHOW COLUMNS FROM students

Lists all the columns from

students

table that is in the

university

database

SHOW COLUMNS FROM university.students

Another method to specify a database when listing columns from a table

SHOW COLUMNS FROM students IN university

Lists all the columns from the specified table

SHOW COLUMNS FROM students

Shortcut of the query above is

DESCRIBE

DESCRIBE students

Lists all the tables from the specified MySQL database

UniversityDB

SHOW TABLES FROM UniversityDB

Lists all the databases on the MySQL host:

SHOW SCHEMAS

Note: The above query is a synonym for

SHOW DATABASES

Shows all the databases that the current user has access to

SHOW DATABASES

The following query is a synonym of the above query. Both of them list all the databases on the MySQL host:

SHOW SCHEMAS

UNION DISTINCT

combines the two results and removes duplicates

SELECT *
FROM students
WHERE id < 10
UNION DISTINCT
SELECT *
FROM students
WHERE id > 5

The query is equivalent to the above query because

DISTINCT

is the default behavior

SELECT *
FROM students
WHERE id < 10
UNION
SELECT *
FROM students
WHERE id > 5

Object

const taskObjects = {
  task: 'code',
  date: 'today',
  repeat: true
}
console .log(taskObjects)
//{task: "code", date: "today", repeat: true}

More "traditional" key/value Easier to write and access values with . and [ ] Use when you need to include

Functions

(methods) Use when working with JSON (can convert to map)

Maps

const taskMaps = new Map( [
   ['task', 'code'],
   ['date', 'today'],
   [false, 'start coding!'],
]);
console .log(taskMaps)
//{"task" => "code", "date" => "today", false => "start coding!"}

Better performance Keys can have any data type Easy to iterate Easy to compute size Use when you simply need to map key to values Use when you need key that are

not

strings