Update sql_ddl.md for Flink SQL table creation with partitioning fix

[YesIfwRONG]

In the previous "Create table with record keys and ordering fields" section, the example created a table with a partitioned column dt that was not defined in the schema. This omission will cause errors when running the SQL code, as Flink SQL requires all partitioned columns to be present in the table schema.

This commit updates sql_ddl.md to include the dt column in the table schema when using PARTITIONED BY (dt), ensuring that the example can be executed without errors. Additionally, it clarifies that all partition columns must exist in the schema to prevent similar issues.

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