Pitutur endi cara SELECT statement bisa tindak?

Question

Accepted Answer

**`SELECT`** ngambil data saka siji utawa luwih tabel — iku statement SQL sing paling asring dienggo. Sampeyan nyatakke kolom endi sing pengin lan saka tabel endi, pilihan kanggo nyaring, ngrucut, lan mbatesi asile.

## SELECT Dasar

```sql
SELECT name, email              -- specific columns
FROM users;

SELECT *                        -- ALL columns (convenient, but avoid in production code)
FROM users;

SELECT name AS full_name        -- AS gives a column an alias (renamed in the output)
FROM users;
```

Sampeyan mendhelepake kolom sing bakal dibaleni sawisih `SELECT` (utawa `*` kanggo kabeh), lan tabel sawisih `FROM`. Alias (`AS`) ngowahi jeneng kolom ing asile.

## Aja ngenggo SELECT * ing kode aplikasi

```sql
-- ❌ SELECT * fetches every column — wasteful, fragile (breaks if columns change)
SELECT * FROM users;
-- ✅ select only what you need — less data transferred, clearer intent, more stable
SELECT id, name, email FROM users;
```

Dene `SELECT *` gampang kanggo eksplorasi, ing kode nyata sampeyan kudu milih kolom sing sampeyan butuhake — iku ngirim data luwih sitik, bisa nggawa maksud jelas, lan ora bakal rusak yen kolom tabel owah.

## Kolom sing dihitung lan ekspresi

```sql
SELECT
  name,
  price * quantity AS total,       -- a calculated column
  UPPER(name) AS name_upper,        -- a function applied to a column
  price * 1.1 AS price_with_tax
FROM orders;
```

`SELECT` bisa isine ekspresi lan fungsi, ora mung kolom polos.

## Urutan logis saka query

```sql
SELECT name                      -- 5. select columns (runs near-last logically)
FROM users                       -- 1. from the table
WHERE active = true              -- 2. filter rows
GROUP BY country                 -- 3. group
HAVING COUNT(*) > 5              -- 4. filter groups
ORDER BY name                    -- 6. sort
LIMIT 10;                        -- 7. limit
```

```text
LOGICAL execution order (NOT the written order):
  FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY → LIMIT
→ This is why you can't use a SELECT alias in WHERE (WHERE runs before SELECT).
```

## Kangapa iku pentinge

`SELECT` iku fondasi saka panyulikane data — statement SQL sing paling asring dienggo, pusat kanggo maca saka database relasional apa wae, dadi nggawase iku kabutuhan saben dina.

Sapa weruh carane nyatakke kolom, ngenggo alias, lan kalebu ekspresi sing dihitung, iku nutupi dasar-dasare ngambil lan nggawé bentuke data.

Praktik sing penting yaiku **aja ngenggo `SELECT *` ing kode aplikasi** (pilih kolom sing dibutuhake ngirim data luwih sitik, luwih jelas, lan luwih kuwat marang owah-owahan skema — praktek sing umum banget).

Sang penting yaiku paham **urutan eksekusi logis** saka query (FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY → LIMIT), sing beda saka urutan sing ditulis lan nerangake kelakuan nyata — kaya mangapa sampeyan ora bisa ngreferensi alias `SELECT` ing klausa `WHERE` (amarga WHERE logis mlaku sadurunge SELECT).

Model mental iki ngenani carane query bener-bener ngolah iku dasar kanggo nulis query sing bener lan debug kesalahan sing bingung.

Amarga maca data nganggo `SELECT` iku operasi database sing paling umum, lan amarga paham pilihan kolom, alias, ekspresi, lan utamane urutan query logis iku dasar kanggo kabeh panyulikane, ngawasi statement `SELECT` iku kabutuhan inti, kabutuhan sing kudu diduweni sapa waé sing ng-SQL database.