You should not use an expression directly in the query. Instead assign the query in an expression to a variable, then use the variable as the query.
For example something like:
<set-variable value="#['SELECT * FROM EMPLOYEE_TABLE WHERE 1=1 ' ++ payload]" name="query" />
<db:select ...>
<db:sql>#[vars.query]</db:sql>
</db:select>
Note that you risk having SQL injections vulnerabilities doing this.
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