<a id="ksqldb-reference-operators"></a>

# Operators in ksqlDB for Confluent Platform

## Expression syntax

ksqlDB supports the following syntax to define WHERE expressions.

```none
expr:
    expr AND expr
  | expr OR expr
  | NOT expr
  | predicate

predicate:
    value_expr comparison_operator value_expr
  | value_expr [NOT] BETWEEN value_expr AND value_expr
  | value_expr [NOT] LIKE value_expr [ESCAPE string]
  | value_expr IS [NOT] NULL
  | value_expr IS [NOT] DISTINCT FROM value_expr

comparison_operator: EQ | NEQ | LT | LTE | GT | GTE

value_expr:
    primary_expr
  | + value_expr
  | - value_expr
  | value_expr + value_expr
  | value_expr - value_expr
  | value_expr * value_expr
  | value_expr / value_expr
  | value_expr % value_expr
  | value_expr AT timezone
  | value_expr CONCAT value_expr

primary_expr:
    literal
  | identifier
  | function_call
  | case_expr
  | cast_expr

arithmetic_operator: + | - | * | / | %
```

## Operators

ksqlDB supports the following operators in value expressions.

### Arithmetic

The usual arithmetic operators (`+,-,/,*,%`) may be applied to numeric
types, like INT, BIGINT, and DOUBLE:

```sql
SELECT USERID, LEN(FIRST_NAME) + LEN(LAST_NAME) AS NAME_LENGTH FROM USERS EMIT CHANGES;
```

### Concatenation

The concatenation operator (`+,||`) can be used to concatenate STRING
values.

```sql
SELECT USERID, FIRST_NAME + LAST_NAME AS FULL_NAME FROM USERS EMIT CHANGES;
```

You can use the `+` operator for multi-part concatenation, for
example:

```sql
SELECT USERID,
    TIMESTAMPTOSTRING(ROWTIME, 'yyyy-MM-dd HH:mm:ss') +
        ': :heavy_exclamation_mark: On ' +
        HOST +
        ' there were ' +
        CAST(INVALID_LOGIN_COUNT AS VARCHAR) +
        ' attempts in the last minute (threshold is >=4)'
  FROM INVALID_USERS_LOGINS_PER_HOST
  WHERE INVALID_LOGIN_COUNT>=4
  EMIT CHANGES;
```

### Source dereference

The source dereference operator (`.`) can be used to specify columns
by dereferencing the source stream or table.

```sql
SELECT USERID, USERS.FIRST_NAME FROM USERS EMIT CHANGES;
```

### Subscript

The subscript operator (`[subscript_expr]`) is used to reference the
value at an array index or a map key.

```sql
SELECT USERID, NICKNAMES[1] FROM USERS EMIT CHANGES;
```

### STRUCT dereference

Access nested data by declaring a STRUCT and using the dereference
operator (`->`) to access its fields:

```sql
SELECT USERID, ADDRESS->STREET, ADDRESS->HOUSE_NUM FROM USERS EMIT CHANGES;
```

Access all STRUCT fields by using a `*` in the dereference operator:

```sql
SELECT USERID, ADDRESS->* FROM USERS EMIT CHANGES;
```

Combine `->` with `.` when using aliases:

```sql
SELECT USERID, USERS.ADDRESS->STREET, U.ADDRESS->STREET FROM USERS U EMIT CHANGES;
```

For more information on nested data, see
[STRUCT](../../reference/sql/data-types.md#ksqldb-reference-data-types-struct).