KSQL is generally case-sensitive. Very sensitive, at times ;-)

Topics 🔗

Topics need to be quoted, always.

ksql> LIST TOPICS;

 Kafka Topic            | Registered | Partitions | Partition Replicas | Consumers | ConsumerGroups
----------------------------------------------------------------------------------------------------
 localpcap              | true       | 1          | 1                  | 2         | 2

ksql> PRINT localpcap;
Could not find topic 'LOCALPCAP', KSQL uses uppercase.
To print a case-sensitive topic apply quotations, for example: print 'topic';
ksql> PRINT 'localpcap';
{"ROWTIME":1548071548370,"ROWKEY":"null","index":{"_index":"packets-2019-01-21","_type":"pcap_file"}}
…
ksql> CREATE STREAM FOO (COL1 VARCHAR) \
        WITH (KAFKA_TOPIC='localPCAP', VALUE_FORMAT='JSON');
Kafka topic does not exist: localPCAP

ksql> CREATE STREAM FOO (COL1 VARCHAR) \
        WITH (KAFKA_TOPIC='localpcap', VALUE_FORMAT='JSON');

 Message
----------------
 Stream created
----------------

Statements 🔗

Statements are not case-sensitive:

ksql> LisT toPICs;

 Kafka Topic            | Registered | Partitions | Partition Replicas | Consumers | ConsumerGroups
----------------------------------------------------------------------------------------------------
 localpcap              | true       | 1          | 1                  | 2         | 2

Object names 🔗

This gets a bit more complicated. TL;DR is that KSQL will force unquoted names to uppercase. Thus, these statements all succeed since the object itself is uppercase:

ksql> LIST STREAMS;

 Stream Name     | Kafka Topic     | Format
--------------------------------------------
 SOURCE          | SOURCE          | JSON
--------------------------------------------

ksql> SELECT * FROM source LIMIT 1;
1548071546868 | null | null
Limit Reached
Query terminated

ksql> SELECT * FROM SOURCE LIMIT 1;
1548071546868 | null | 0
Limit Reached
Query terminated

ksql> SELECT * FROM souRCE LIMIT 1;
1548071546868 | null | null
Limit Reached
Query terminated

However, this mixed-case object name must be quoted when referenced:

ksql> LIST STREAMS;

 Stream Name     | Kafka Topic     | Format
--------------------------------------------
 MixedCaseStream | MixedCaseStream | JSON

ksql> SELECT COL1 FROM MixedCaseStream LIMIT 1;
Failed to prepare statement: MIXEDCASESTREAM does not exist.
Caused by: MIXEDCASESTREAM does not exist.

ksql> SELECT COL1 FROM "MixedCaseStream" LIMIT 1;
null
Limit Reached
Query terminated
ksql>

A slight twist on this is that SELECT * doesn’t work, but specifying the individual columns does (#2176):

ksql> DESCRIBE "MixedCaseStream";

Name                 : MixedCaseStream
 Field                    | Type
------------------------------------------------------
 ROWTIME                  | BIGINT           (system)
 ROWKEY                   | VARCHAR(STRING)  (system)
 COL1                     | VARCHAR(STRING)
 lowercase_col1           | VARCHAR(STRING)
 MixedCase_COL1           | VARCHAR(STRING)
 UPPERCASE_COL1           | VARCHAR(STRING)
 NOTQUOTED_MIXEDCASE_COL1 | VARCHAR(STRING)
------------------------------------------------------
For runtime statistics and query details run: DESCRIBE EXTENDED <Stream,Table>;
ksql> SELECT * FROM "MixedCaseStream";
Invalid Expression MixedCaseStream.ROWTIME.
ksql>
ksql> SELECT ROWTIME, COL1 FROM "MixedCaseStream" LIMIT 1;
1548071546868 | null
Limit Reached
Query terminated

And now we get to the crux of it, which is that every non-uppercase object and column needs quoting:

ksql> SELECT ROWTIME, ROWKEY, COL1, lowercase_col1, \
             MixedCase_COL1, UPPERCASE_COL1, \
             NOTQUOTED_MIXEDCASE_COL1 \
        FROM "MixedCaseStream" LIMIT 1;
Column LOWERCASE_COL1 cannot be resolved.
ksql>
ksql> SELECT ROWTIME, ROWKEY, COL1, "lowercase_col1", \
             "MixedCase_COL1", UPPERCASE_COL1, \
             NOTQUOTED_MIXEDCASE_COL1 \
        FROM "MixedCaseStream" LIMIT 1;
1548071547366 | null | null | null | null | null | null
Limit Reached
Query terminated

Creating Mixed-Case objects and columns 🔗

TL;DR : enclose them in quotes.

ksql> CREATE STREAM "MixedCaseStream" AS \
            SELECT COL1, \
                   COL1 AS "lowercase_col1", \
                   COL1 AS "MixedCase_COL1", \
                   COL1 AS "UPPERCASE_COL1", \
                   COL1 AS NOTQUOTED_MixedCase_COL1 \
          FROM SOURCE;

 Message
----------------------------
 Stream created and running
----------------------------

BUT as shown above, once created as a non-uppercase column, you need to forever-after quote it when referencing it.