Code example conventions

Note about the code examples

The functionality of each operator and function is illustrated by a code example of this form:

[function or operator]
  acts on
[argument (list)]
  to produce
[result value]

It's insufficient to present such examples in the style "run this SQL at the ysqlsh prompt" followed by "see this result". A SQL-only demonstration has these disadvantages:

  • It's hard to establish and advertise data type information without inserting into appropriately defined table columns. This brings distracting verbosity.
  • The only way to build an expression from subexpressions, in pursuit of clarity, is to use scalar subqueries, named in a with clause. This, again, is so verbose that it obscures, rather than helps, clarity.
  • This insufficiency is especially bothersome when the aim is to show how the output of a particular function depends upon the choice of value for an optional boolean parameter that conditions is behavior using, therefore, the same input JSON value in two function invocations.
  • The result is a non-negotiable typecast to text to print to the screen, with additional distracting conventions like, for example, inserting a space at the start of each printed line, showing the boolean value TRUE as the text value t, showing NULL as just an absence, and showing a newline as the text value +.

For these reasons, each code example is presented as a DO block with this pattern:

  • Each input value is declared using the appropriate SQL data type (sometimes building such values bottom-up from declared simpler values).
  • The output of the operator or expression is assigned to a variable of the appropriate data type.
  • The expected output is declared as a value of the same data type.
  • An ASSERT is used to show that the produced value is equal to the expected value, using an IS NULL comparison where appropriate.

Note about SQL array literals

RFC 7159 defines the syntax for a JSON array as a comma-separated list of items surrounded by [] and the syntax for a JSON object as a comma-separated list of key-value pairs surrounded by {}. The literal for a SQL array is a text value with an inner syntax, typecast the array's data type: the value starts with { and ends with } and contains a comma-separated list whose items are not themselves single-quoted but are all taken to be values of the array's data type. So this SQL array value:

array['a', 'b', 'c']::text[]

can also be written thus:

'{a, b, c}'::text[]

See the section Creating an array value using a literal for more information on this topic. This dramatic context-sensitive difference in meaning of '{...}' might confuse the reader. Therefore, in the major section "JSON data types and functionality", the array[...] constructor form will be used for a SQL array value—and the use of the '{...}' SQL array literal will be avoided.

The fact that a JSON array can have subvalues of mixed data type but a SQL array can have only elements of the same data type means that special steps have to be taken when the goal is to construct a JSON array mixed subvalue data type from SQL values.