# PostgreSQL Quick Reference

*Tables, queries, joins, indexes, JSON, roles*

> Source: PostgreSQL Documentation (postgresql.org/docs) · MIT

## Connecting

### Command Line

```
psql -U postgres
psql -h localhost -p 5432 -U user -d mydb
psql "postgresql://user:pass@host:5432/mydb"
```

### psql Meta-Commands

| Command | Description |
|---------|-------------|
| `\l` | List databases |
| `\c dbname` | Connect to database |
| `\dt` | List tables |
| `\d tablename` | Describe table structure |
| `\dn` | List schemas |
| `\du` | List roles |
| `\q` | Quit psql |
| `\i file.sql` | Execute SQL file |

## Tables & Schemas

### Create Table

```
CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  name VARCHAR(100) NOT NULL,
  email TEXT UNIQUE,
  created_at TIMESTAMPTZ DEFAULT NOW()
);
```

### Schema Operations

```
CREATE SCHEMA app;
CREATE TABLE app.users (id SERIAL PRIMARY KEY);
SET search_path TO app, public;
DROP SCHEMA app CASCADE;
```

### Alter Table

```
ALTER TABLE users ADD COLUMN age INT;
ALTER TABLE users ALTER COLUMN name TYPE TEXT;
ALTER TABLE users DROP COLUMN age;
ALTER TABLE users RENAME TO customers;
```

## Data Types

### Numeric

| Command | Description |
|---------|-------------|
| `INTEGER / INT` | 4-byte integer |
| `BIGINT` | 8-byte integer |
| `SERIAL` | Auto-incrementing integer |
| `NUMERIC(p,s)` | Exact numeric (e.g., NUMERIC(10,2)) |
| `REAL / DOUBLE PRECISION` | Floating-point (4 / 8 bytes) |
| `BOOLEAN` | true / false / null |

### String & Binary

| Command | Description |
|---------|-------------|
| `TEXT` | Variable unlimited text |
| `VARCHAR(n)` | Variable text up to n chars |
| `CHAR(n)` | Fixed-length text |
| `BYTEA` | Binary data |
| `UUID` | 128-bit universally unique id |

### Date, JSON & Array

| Command | Description |
|---------|-------------|
| `DATE` | Calendar date |
| `TIMESTAMPTZ` | Timestamp with time zone |
| `INTERVAL` | Time span (e.g., '2 days') |
| `JSONB` | Binary JSON (indexable) |
| `INT[] / TEXT[]` | Array types |

## Queries

### Insert

```
INSERT INTO users (name, email)
VALUES ('Alice', 'alice@example.com')
RETURNING id;

INSERT INTO users (name, email) VALUES
('Bob', 'bob@ex.com'),
('Carol', 'carol@ex.com');
```

### Select

```
SELECT * FROM users WHERE id = 1;
SELECT name, email FROM users
ORDER BY name LIMIT 10 OFFSET 20;
```

### Update

```
UPDATE users SET email = 'new@ex.com'
WHERE id = 1 RETURNING *;
```

### Upsert

```
INSERT INTO users (email, name)
VALUES ('a@ex.com', 'Alice')
ON CONFLICT (email) DO UPDATE
SET name = EXCLUDED.name;
```

### Delete

```
DELETE FROM users WHERE id = 1 RETURNING *;
TRUNCATE TABLE users RESTART IDENTITY;
```

## Joins & Subqueries

### Join Types

| Command | Description |
|---------|-------------|
| `INNER JOIN` | Rows matching in both tables |
| `LEFT JOIN` | All left rows + matching right |
| `RIGHT JOIN` | All right rows + matching left |
| `FULL OUTER JOIN` | All rows from both tables |
| `CROSS JOIN` | Cartesian product |
| `LATERAL JOIN` | Subquery referencing outer row |

### CTE (Common Table Expression)

```
WITH active AS (
  SELECT * FROM users WHERE active = true
)
SELECT a.name, o.total
FROM active a
JOIN orders o ON a.id = o.user_id;
```

### Subquery

```
SELECT name FROM users
WHERE id IN (
  SELECT user_id FROM orders
  WHERE total > 100
);
```

## Indexes

### Create & Drop

```
CREATE INDEX idx_name ON users(name);
CREATE UNIQUE INDEX idx_email ON users(email);
CREATE INDEX idx_gin ON posts USING GIN(tags);
DROP INDEX idx_name;
```

### Index Types

| Command | Description |
|---------|-------------|
| `B-tree` | Default, good for =, <, >, BETWEEN |
| `Hash` | Equality comparisons only |
| `GIN` | Generalized inverted — arrays, JSONB, full-text |
| `GiST` | Generalized search — geometric, range |
| `BRIN` | Block range — large sorted tables |

### Query Analysis

```
EXPLAIN ANALYZE
SELECT * FROM users WHERE name = 'Alice';
```

## Functions & Procedures

### SQL Function

```
CREATE FUNCTION active_count()
RETURNS INTEGER AS $$
  SELECT COUNT(*)::INT FROM users
  WHERE active = true;
$$ LANGUAGE sql;
SELECT active_count();
```

### PL/pgSQL Function

```
CREATE FUNCTION greet(name TEXT)
RETURNS TEXT AS $$
BEGIN
  RETURN 'Hello, ' || name;
END;
$$ LANGUAGE plpgsql;
```

### Useful Built-ins

| Command | Description |
|---------|-------------|
| `NOW() / CURRENT_TIMESTAMP` | Current timestamp with TZ |
| `AGE(ts1, ts2)` | Interval between timestamps |
| `COALESCE(a, b)` | First non-null value |
| `NULLIF(a, b)` | NULL if a = b |
| `GENERATE_SERIES(1,10)` | Generate rows of sequential values |
| `STRING_AGG(col, ',')` | Concatenate values with separator |

## Roles & Permissions

### Role Management

```
CREATE ROLE app LOGIN PASSWORD 'secret';
ALTER ROLE app SET search_path TO myapp;
DROP ROLE app;
```

### Grants

```
GRANT ALL ON DATABASE mydb TO app;
GRANT SELECT, INSERT ON users TO reader;
GRANT USAGE ON SCHEMA public TO app;
REVOKE INSERT ON users FROM reader;
```

### Row-Level Security

```
ALTER TABLE users ENABLE ROW LEVEL SECURITY;
CREATE POLICY user_own ON users
FOR ALL USING (id = current_setting('app.uid')::INT);
```

## JSON Support

### JSONB Operators

| Command | Description |
|---------|-------------|
| `->'key'` | Get JSON value by key (as JSON) |
| `->>'key'` | Get JSON value by key (as text) |
| `#>'{a,b}'` | Get nested value by path |
| `@>` | Contains (left includes right) |
| `?` | Key exists |
| `\|\|` | Concatenate JSONB values |

### JSONB Queries

```
SELECT data->>'name' FROM profiles
WHERE data @> '{"active": true}';

SELECT * FROM profiles
WHERE data ? 'email';
```

### JSONB Functions

```
SELECT jsonb_each(data) FROM profiles;
SELECT jsonb_array_elements('[1,2,3]');
SELECT jsonb_set(data, '{name}', '"Alice"')
FROM profiles WHERE id = 1;
```

## Common Patterns

### Transactions

```
BEGIN;
UPDATE accounts SET balance = balance - 100
WHERE id = 1;
UPDATE accounts SET balance = balance + 100
WHERE id = 2;
COMMIT;  -- or ROLLBACK;
```

### Window Functions

```
SELECT name, salary,
  RANK() OVER (ORDER BY salary DESC),
  AVG(salary) OVER (PARTITION BY dept)
FROM employees;
```

### Copy Data

```
COPY users TO '/tmp/users.csv'
WITH (FORMAT csv, HEADER);
COPY users FROM '/tmp/users.csv'
WITH (FORMAT csv, HEADER);
```

### pg_dump Backup

```
pg_dump -U postgres mydb > backup.sql
pg_dump -Fc mydb > backup.dump
pg_restore -d mydb backup.dump
```