Columns and Data Types

@Column

@Column(name = "product_name", nullable = false, length = 150)
private String name;

@Column(length = 2000)
private String description;

@Column(nullable = false, precision = 10, scale = 2)     // up to 99,999,999.99
private BigDecimal price;

@Column(nullable = false)
private Integer stock = 0;

@Column(unique = true, nullable = false, updatable = false, length = 40)
private String sku;

@Column(name = "created_at", nullable = false, updatable = false)
private LocalDateTime createdAt;

Attribute	Meaning
`name`	Column name (default: field name in snake_case with Spring Boot)
`nullable`	`false` adds `not null` to the DDL
`length`	`varchar` size for strings (default 255)
`precision`, `scale`	Total digits and digits after the decimal point for `BigDecimal`
`unique`	Adds a unique constraint
`insertable`, `updatable`	`false` = Hibernate never writes this column in INSERT / UPDATE

Note: nullable, length and unique only affect the generated DDL. They do not validate data in Java. Use Bean Validation (@NotBlank, @Size) for that — see the Spring Boot material.

Java Type to SQL Type

Java type	MySQL	PostgreSQL
`Long` / `long`	`bigint`	`bigint`
`Integer` / `int`	`int`	`integer`
`String`	`varchar(255)`	`varchar(255)`
`BigDecimal`	`decimal(p,s)`	`numeric(p,s)`
`Boolean`	`bit`	`boolean`
`LocalDate`	`date`	`date`
`LocalDateTime`	`datetime(6)`	`timestamp(6)`
`Instant`, `OffsetDateTime`	`datetime(6)`	`timestamp(6) with time zone`
`UUID`	`binary(16)`	`uuid`
`byte[]` with `@Lob`	`longblob`	`bytea` / `oid`

Money: always BigDecimal, never double (0.1 + 0.2 is not 0.3 in floating point).
Dates: use java.time types. The old java.util.Date with @Temporal is no longer needed.
Wrapper vs primitive: Integer can hold null (unknown); int cannot. Prefer wrappers for ids so a new entity has id == null.

Why BigDecimal for Prices

Example

import java.math.BigDecimal;

public class Main {
    public static void main(String[] args) {
        double d = 0.10 + 0.20;
        System.out.println("double:     " + d);

        BigDecimal b = new BigDecimal("0.10").add(new BigDecimal("0.20"));
        System.out.println("BigDecimal: " + b);

        // 3 items at Rs.199.99 with 18% GST
        BigDecimal price = new BigDecimal("199.99");
        BigDecimal total = price.multiply(BigDecimal.valueOf(3))
            .multiply(new BigDecimal("1.18"))
            .setScale(2, java.math.RoundingMode.HALF_UP);
        System.out.println("Total with GST: Rs." + total);
    }
}

Output

double:     0.30000000000000004
BigDecimal: 0.30
Total with GST: Rs.707.96