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Very simple example of a JSON payload: 
{
  "id" : 1,
  "customer_id" : 1234,
  "customer_name" : "John Doe"
}
And the corresponding schema: 
{
  "type" : "struct",
  "fields" : [ {
    "type" : "int64",
    "optional" : false,
    "field" : "id"
  }, {
    "type" : "int64",
    "optional" : false,
    "field" : "customer_id"
  }, {
    "type" : "string",
    "optional" : false,
    "field" : "customer_name"
  } ],
  "optional" : false
}
To make it easier to use the Kafka types, please find below the list of supported types mapped to usual SQL types that might be found in source database.
Kafka TypeSQL Type (PostgreSQL)SQL Type (MySQL)SQL Type (Oracle)
int8SMALLINTTINYINTNUMBER(3)
int16INTEGERSMALLINTNUMBER(5)
int32INTEGERINTNUMBER(10)
int64BIGINTBIGINTNUMBER(19)
floatREALFLOATNUMBER(7, 2)
doubleDOUBLE PRECISIONDOUBLENUMBER(15, 6)
stringVARCHARVARCHARVARCHAR2
bytesBYTEABLOBBLOB
booleanBOOLEANBOOLEANNUMBER(1)
{"type":"int64","optional":false,"name":"org.apache.kafka.connect.data.Timestamp"}TIMESTAMPTIMESTAMPTIMESTAMP
{"type":"bytes","optional":false,"name":"org.apache.kafka.connect.data.Decimal","parameters":{"scale":"4","connect.decimal.precision":"38"}}DECIMAL(38,4)DECIMAL(38,4)DECIMAL(38,4)
{"type":"int32","optional":false,"name":"org.apache.kafka.connect.data.Date"}DATEDATEDATE
{"type":"int64","optional":false,"name":"org.apache.kafka.connect.data.Time"}TIMETIMEN/A
DECIMAL(38,4) means a decimal number with up to 38 digits in total, of which 4 digits can be after the decimal point, one can adjust the precision if needed. Please see the example of primitive Kafka Schema definitions and their corresponding payloads: 
{"schema":{"type":"boolean","optional":false,"default":false},"payload":false}
{"schema":{"type":"int8","optional":false,"default":1},"payload":1}
{"schema":{"type":"int16","optional":false,"default":1},"payload":1}
{"schema":{"type":"int32","optional":false,"default":1},"payload":1}
{"schema":{"type":"int64","optional":false,"default":1},"payload":1}
{"schema":{"type":"float","optional":false,"default":1.0},"payload":1.0}
{"schema":{"type":"double","optional":false,"default":1.0},"payload":1.0}
{"schema":{"type":"bytes","optional":false,"default":"Zm9v"},"payload":"Zm9v"}
{"schema":{"type":"string","optional":false,"default":"foo"},"payload":"foo"}
{"schema":{"type":"bytes","optional":false,"name":"org.apache.kafka.connect.data.Decimal","parameters":{"scale":"4","connect.decimal.precision":"38"},"default":"EtaH"},"payload":"EtaH"}
{"schema":{"type":"int64","optional":false,"name":"org.apache.kafka.connect.data.Timestamp","default":1758635042028},"payload":1758635042028}
{"schema":{"type":"int64","optional":false,"name":"org.apache.kafka.connect.data.Time","default":55800000},"payload":55800000}
{"schema":{"type":"int32","optional":false,"name":"org.apache.kafka.connect.data.Date","default":20370},"payload":20370}