Filtering data with WHERE clause

Hai semuanya, di materi kali ini kita akan membahas tentang mem-Filter data dengan menggunakan WHERE clause di PostgreSQL. By default jika kita menggunakan perintah select itu akan menampilkan semua data pada suatu atau berberapa table. Dengan menggunakan clause WHERE kita bisa memfilter atau memilih data yang akan kita tampilkan.

Perintah dasar untuk WHERE clause seperti berikut:

select * | column_list ...
from a_table
where search_condition

Dimana search_condition adalah any value expression (functions, operators and predicates ) yang mengembalikan nilai boolean. Sebagai gambaran berikut ilustrasinya:

Ilustrasi tersebut menggambarkan, suatu predicate atau search_condition yaitu menggunakan operator equal pada column status dengan nilai active maka database akan mencari nilainya active saja pada kolom status. Untuk search_condition tersebut biasanya di sebut dengan predicate. Ada banyak sekali predicate yang kita bisa gunakan tapi disini kita bahasnya paling dasar dulu ya yaitu:

1. Relational predicate
2. Like predicates
3. Between predicates
4. Null predicate
5. Logical predicate
6. Regular Expression (Regex) predicate
7. Row and Array Comparisons

Ok kita bahas satu-per-satu ya, mulai dari relational predicate

Relational predicate

Relational predicates pada dasarnya adalah comparison operators yaitu

1. Equal (=)
2. Not Equals (!= or <>)
3. Less than or equal to (< or <=)
4. Greater then or equal to (> or >=)

Contoh penggunaannya seperti berikut:

Contoh lainnya seperti berikut:

Jika dijalankan maka hasilnya seperti berikut:

hr=# select employee_id, first_name, last_name, email, phone_number, job_id, salary
hr-# from employees
hr-# where salary >= 20000;
 employee_id | first_name | last_name | email | phone_number | job_id  |  salary
-------------+------------+-----------+-------+--------------+---------+----------
         100 | Steven     | King      | SKING | 515.123.4567 | AD_PRES | 24000.00
(1 row)

Nah jadi kita kita perhatikan dari hasil query tersebut, kita memfilter menggunakan operator >= atau lebih besar sama dengan hasilnya tidak ada yang salarnya lebih kecil dari kita definisikan yaitu 20_000.

Like predicates

Operator like biasanya digunakan untuk tipe data string (varchar, text), ada 2 expresion yang kita bisa gunakan di operator ini yaitu

1. _ atau underscore, digunakan untuk mewakili expresion satu karakter.
2. % atau persent, digunakan untuk mewakili expresion beberapa karakter.

Contoh kasus untuk expresion %, saya ingin mencari nama depan karyawan yang diawali oleh huruf A. Berikut querynya:

Jika dijalankan hasilnya seperti berikut:

hr=# select employee_id, first_name, last_name, email, phone_number, job_id, salary
hr-# from employees
hr-# where last_name like 'A%';
 employee_id | first_name | last_name |  email   |    phone_number    |  job_id  |  salary
-------------+------------+-----------+----------+--------------------+----------+----------
         105 | David      | Austin    | DAUSTIN  | 590.423.4569       | IT_PROG  |  4800.00
         130 | Mozhe      | Atkinson  | MATKINSO | 650.124.6234       | ST_CLERK |  2800.00
         166 | Sundar     | Ande      | SANDE    | 011.44.1346.629268 | SA_REP   |  6400.00
         174 | Ellen      | Abel      | EABEL    | 011.44.1644.429267 | SA_REP   | 11000.00
(4 rows)

Sedangkan untuk contoh expresion _, saya ingin mencari huruf ke 2 dari kolom job_id di tabel jobs mengadung t. Berikut querynya:

Jika dijalankan hasilnya seperti berikut:

hr=# select employee_id, first_name, last_name, email, phone_number, job_id, salary
hr-# from employees
hr-# where last_name like '_t%';
 employee_id | first_name | last_name |  email   | phone_number |  job_id  | salary
-------------+------------+-----------+----------+--------------+----------+---------
         130 | Mozhe      | Atkinson  | MATKINSO | 650.124.6234 | ST_CLERK | 2800.00
         138 | Stephen    | Stiles    | SSTILES  | 650.121.2034 | ST_CLERK | 3200.00
(2 rows)

Between predicates

Operator BETWEEN digunakan untuk memfilter dengan interval/rentang tertentu diantar nilai terkecil dan terbesar biasanya digunakan untuk tipe data seperti number, date, time, datetime serta char.

Contoh kasusnya, Saya mau menampilkan data yang karywan yang memiliki gaji dari 4000 s/d 6000. Berikut querynya:

Jika dijalankan hasilnya seperti berikut:

hr=# select employee_id, first_name, last_name, email, phone_number, job_id, salary
hr-# from employees
hr-# where salary between 17000 and 20000;
 employee_id | first_name | last_name |  email   | phone_number | job_id |  salary
-------------+------------+-----------+----------+--------------+--------+----------
         101 | Neena      | Kochhar   | NKOCHHAR | 515.123.4568 | AD_VP  | 17000.00
         102 | Lex        | De Haan   | LDEHAAN  | 515.123.4569 | AD_VP  | 17000.00
(2 rows)

Atau berikut contoh lainnya, saya mau mengambil first_name yang mengandung huruf h sampai j pada character ke 2. maka querynya seperti berikut:

Jika dijalankan hasilnya seperti berikut:

hr=# select employee_id, first_name, last_name
hr-# from employees
hr-# where substring(first_name from 2 for 1) between 'h' and 'j';
 employee_id | first_name  | last_name
-------------+-------------+-----------
         107 | Diana       | Lorentz
         116 | Shelli      | Baida
         117 | Sigal       | Tobias
         205 | Shelley     | Higgins
         206 | William     | Gietz
(18 rows)

Null predicate

Operator IS NULL digunakan untuk memfiter data yang bernilai null.

Contoh kasusnya, saya mau menampilkan data karywan yang tidak memiliki manager. Berikut querynya:

Jika dijalankan hasilnya seperti berikut:

hr=# select employee_id, first_name, last_name, job_id, manager_id
hr-# from employees
hr-# where manager_id is null;
 employee_id | first_name | last_name | job_id  | manager_id
-------------+------------+-----------+---------+------------
         100 | Steven     | King      | AD_PRES |
(1 row)

Logical predicate

Logical predicate pada dasarnya sama seperti logical operators yaitu

1. and operators
2. or operators
3. not operators

Kasusnya saya mau mencari data karyawan yang berkerja di department_id = 90 dan yang manager_id = 100 berarti kita bisa menggunakan operator and, berikut querynya:

Jika kita jalankan maka hasilnya seperti berikut:

hr=# select employee_id, first_name, last_name, department_id, manager_id
hr-# from employees
hr-# where department_id = 90 and manager_id = 100;
 employee_id | first_name | last_name | department_id | manager_id
-------------+------------+-----------+---------------+------------
         101 | Neena      | Kochhar   |            90 |        100
         102 | Lex        | De Haan   |            90 |        100
(2 rows)

Dan sedangkan untuk operator or seperti berikut:

Jika kita jalankan maka hasilnya seperti berikut:

hr=# select employee_id, first_name, last_name, department_id, manager_id
hr-# from employees
hr-# where department_id = 90 or manager_id = 100;
 employee_id | first_name | last_name | department_id | manager_id
-------------+------------+-----------+---------------+------------
         100 | Steven     | King      |            90 |
         101 | Neena      | Kochhar   |            90 |        100
         102 | Lex        | De Haan   |            90 |        100
         114 | Den        | Raphaely  |            30 |        100
         120 | Matthew    | Weiss     |            50 |        100
         121 | Adam       | Fripp     |            50 |        100
         122 | Payam      | Kaufling  |            50 |        100
         123 | Shanta     | Vollman   |            50 |        100
         124 | Kevin      | Mourgos   |            50 |        100
         145 | John       | Russell   |            80 |        100
         146 | Karen      | Partners  |            80 |        100
         147 | Alberto    | Errazuriz |            80 |        100
         148 | Gerald     | Cambrault |            80 |        100
         149 | Eleni      | Zlotkey   |            80 |        100
         201 | Michael    | Hartstein |            20 |        100
(15 rows)

Dan yang terakhir kita juga bisa menggunakan not operator seperti berikut:

Jika di jalankan maka hasilnya sebagai berikut:

hr=# select employee_id, first_name, last_name, job_id, manager_id, salary
hr-# from employees
hr-# where salary not between 3000 and 20000;
 employee_id | first_name |  last_name  |  job_id  | manager_id |  salary
-------------+------------+-------------+----------+------------+----------
         100 | Steven     | King        | AD_PRES  |            | 24000.00
         116 | Shelli     | Baida       | PU_CLERK |        114 |  2900.00
         117 | Sigal      | Tobias      | PU_CLERK |        114 |  2800.00
         118 | Guy        | Himuro      | PU_CLERK |        114 |  2600.00
         119 | Karen      | Colmenares  | PU_CLERK |        114 |  2500.00
         126 | Irene      | Mikkilineni | ST_CLERK |        120 |  2700.00
         127 | James      | Landry      | ST_CLERK |        120 |  2400.00
         128 | Steven     | Markle      | ST_CLERK |        120 |  2200.00
(25 rows)

Regular Expression (Regex) predicate

The SIMILAR TO operator returns true or false depending on whether its pattern matches the given string. It is similar to LIKE, except that it interprets the pattern using the SQL standard’s definition of a regular expression.

In addition to these facilities borrowed from LIKE, SIMILAR TO supports these pattern-matching metacharacters borrowed from POSIX regular expressions:

1. | denotes alternation (either of two alternatives).
2. * denotes repetition of the previous item zero or more times.
3. + denotes repetition of the previous item one or more times.
4. ? denotes repetition of the previous item zero or one time.
5. {m} denotes repetition of the previous item exactly m times.
6. {m,} denotes repetition of the previous item m or more times.
7. {m,n} denotes repetition of the previous item at least m and not more than n times.
8. () can be used to group items into a single logical item.
9. [...] specifies a character class, just as in POSIX regular expressions.

Some examples:

'abc' SIMILAR TO 'abc'          true
'abc' SIMILAR TO 'a'            false
'abc' SIMILAR TO '%(b|d)%'      true
'abc' SIMILAR TO '(b|c)%'       false
'-abc-' SIMILAR TO '%\mabc\M%'  true
'xabcy' SIMILAR TO '%\mabc\M%'  false

Salah satu penggunaanya seperti berikut:

Jika di jalankan maka hasilnya seperti berikut:

hr=# select employee_id, first_name, phone_number, job_id
hr-# from employees
hr-# where first_name similar to 'Ste(v|ph)en';
 employee_id | first_name | phone_number |  job_id
-------------+------------+--------------+----------
         100 | Steven     | 515.123.4567 | AD_PRES
         128 | Steven     | 650.124.1434 | ST_CLERK
         138 | Stephen    | 650.121.2034 | ST_CLERK
(3 rows)

Selain menggunakan SIMILAR TO Operators kita juga bisa menggunakan POSIX regular expression yang lebih powerfull dibadingkan LIKE dan SIMILAR TO. Many Unix tools such as egrep, sed, or awk use a pattern matching language that is similar to the one described here.

Some examples:

'abcd' ~ 'bc'     true
'abcd' ~ 'a.c'    true -- dot matches any character
'abcd' ~ 'a.*d'   true -- * repeats the preceding pattern item
'abcd' ~ '(b|x)'  true -- | means OR, parentheses group
'abcd' ~ '^a'     true -- ^ anchors to start of string
'abcd' ~ '^(b|c)' false -- would match except for anchoring

Salah satu penggunaanya seperti berikut:

Jika di jalankan maka hasilnya seperti berikut:

hr=# select employee_id, first_name, phone_number, job_id
hr-# from employees
hr-# where first_name ~ '^S.*(a|v|ph)';
 employee_id | first_name |    phone_number    |  job_id
-------------+------------+--------------------+----------
         100 | Steven     | 515.123.4567       | AD_PRES
         117 | Sigal      | 515.127.4564       | PU_CLERK
         123 | Shanta     | 650.123.4234       | ST_MAN
         128 | Steven     | 650.124.1434       | ST_CLERK
         138 | Stephen    | 650.121.2034       | ST_CLERK
         161 | Sarath     | 011.44.1345.529268 | SA_REP
         166 | Sundar     | 011.44.1346.629268 | SA_REP
         173 | Sundita    | 011.44.1343.329268 | SA_REP
         192 | Sarah      | 650.501.1876       | SH_CLERK
         194 | Samuel     | 650.501.3876       | SH_CLERK
         203 | Susan      | 515.123.7777       | HR_REP
(11 rows)

Row and Array Comparisons

This section describes several specialized constructs for making multiple comparisons between groups of values. These forms are syntactically related to the subquery forms but this time we do not involve subqueries. The forms involving array subexpressions are PostgreSQL extensions; the rest are SQL-compliant. All of the expression forms documented in this section return Boolean (true/false) results.

1. IN
2. ANY / SOME
3. ALL

Pertama kita bahas yang paling basic dulu yaitu IN operator,

expression IN (value [, ...])

The right-hand side is a parenthesized list of scalar expressions. This is a shorthand notation for

expression = value1 OR
expression = value2 OR
expression = value3 OR
...

Salah satu penggunaanya seperti berikut:

Jika di jalankan maka hasilnya seperti berikut:

hr=# select employee_id, first_name, phone_number, job_id
hr-# from employees
hr-# where job_id in ('IT_PROG', 'SA_MAN', 'MK_MAN');
 employee_id | first_name |    phone_number    | job_id
-------------+------------+--------------------+---------
         103 | Alexander  | 590.423.4567       | IT_PROG
         104 | Bruce      | 590.423.4568       | IT_PROG
         105 | David      | 590.423.4569       | IT_PROG
         106 | Valli      | 590.423.4560       | IT_PROG
         107 | Diana      | 590.423.5567       | IT_PROG
         145 | John       | 011.44.1344.429268 | SA_MAN
         146 | Karen      | 011.44.1344.467268 | SA_MAN
         147 | Alberto    | 011.44.1344.429278 | SA_MAN
         148 | Gerald     | 011.44.1344.619268 | SA_MAN
         149 | Eleni      | 011.44.1344.429018 | SA_MAN
         201 | Michael    | 515.123.5555       | MK_MAN
(11 rows)

Kemudian kita bahas untuk SOME dan ANY,

expression operator ANY (array expression)
expression operator SOME (array expression)

The right-hand side is a parenthesized expression, which must yield an array value. The left-hand expression is evaluated and compared to each element of the array using the given operator, which must yield a Boolean result.

The simple example is = ANY look like this:

Jika di jalankan maka hasilnya seperti berikut:

hr=# select employee_id, first_name, phone_number, job_id
hr-# from employees
hr-# where job_id = any (ARRAY['IT_PROG', 'SA_MAN', 'MK_MAN']);
 employee_id | first_name |    phone_number    | job_id
-------------+------------+--------------------+---------
         103 | Alexander  | 590.423.4567       | IT_PROG
         104 | Bruce      | 590.423.4568       | IT_PROG
         105 | David      | 590.423.4569       | IT_PROG
         106 | Valli      | 590.423.4560       | IT_PROG
         107 | Diana      | 590.423.5567       | IT_PROG
         145 | John       | 011.44.1344.429268 | SA_MAN
         146 | Karen      | 011.44.1344.467268 | SA_MAN
         147 | Alberto    | 011.44.1344.429278 | SA_MAN
         148 | Gerald     | 011.44.1344.619268 | SA_MAN
         149 | Eleni      | 011.44.1344.429018 | SA_MAN
         201 | Michael    | 515.123.5555       | MK_MAN
(11 rows)

Untuk operator ALL dan lain-lainnya nanti kita bahas selanjutnya di Subquery ya.

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