Cypher Merge or other nGQL to property-wise conditionally insert

[wey-gu]

Is your feature request related to a problem? Please describe.
Similar use case when using MERGE in Cypher.

If we would like to query in one shoot(instead of two queries) to create a user tag vertex only when its email property was not associated with any existing user vertices.

Describe the solution you'd like
Some expression to enable conditionally insert? The easy way/ugly may be to:

• a) add parsing rule on variable support in INSERT VERTEX?
• b) add pipe support in INSERT VERTEX?

(root@nebula) [basketballplayer]> $var = LOOKUP ON user WHERE user.email == "Rudy.Gay@domain.tld" YIELD user.email as email | \
                               ->        GROUP BY $-.email YIELD $-.email as email, count(*) AS number | \
                               ->        YIELD CASE $-.number == 0 WHEN true THEN "user200" ELSE "userdummy" END AS user_vid;
+-------------+
| user_vid    |
+-------------+
| "userdummy" |
+-------------+
Got 1 rows (time spent 4169/44468 us)

a)

$var = LOOKUP ON user WHERE user.email == "Rudy.Gay@domain.tld" YIELD user.email as email | \
       GROUP BY $-.email YIELD $-.email as email, count(*) AS number | \
       YIELD CASE $-.number == 0 WHEN true THEN "user200" ELSE "userdummy" END AS user_vid;
insert vertex user(email) values $var.user_vid:("Rudy.Gay@domain.tld")

(root@nebula) [basketballplayer]> $var = LOOKUP ON user WHERE user.email == "Rudy.Gay@domain.tld" YIELD user.email as email | \
                               ->        GROUP BY $-.email YIELD $-.email as email, count(*) AS number | \
                               ->        YIELD CASE $-.number == 0 WHEN true THEN "user200" ELSE "userdummy" END AS user_vid; \
                               -> insert vertex user(email) values $var.user_vid:("Rudy.Gay@domain.tld")
[ERROR (-1004)]: SyntaxError: syntax error near `$var'

Mon, 19 Jul 2021 17:37:29 CST

b)

(root@nebula) [basketballplayer]> LOOKUP ON user WHERE user.email == "Rudy.Gay@domain.tld" YIELD user.email as email | \
                               ->        GROUP BY $-.email YIELD $-.email as email, count(*) AS number | \
                               ->        YIELD CASE $-.number == 0 WHEN true THEN "user200" ELSE "userdummy" END AS user_vid |\
                               -> insert vertex user(email) values $-.user_vid:("Rudy.Gay@domain.tld")
[ERROR (-1004)]: SyntaxError: syntax error near `insert'

Mon, 19 Jul 2021 17:43:05 CST

Describe alternatives you've considered
Cypher MERGE support? or other smart expressions to enable the condition based on the property of an insert.

Additional context

https://discuss.nebula-graph.com.cn/t/topic/3457

https://nebulagraph.slack.com/archives/CJNFUM7AR/p1626558554041700

[Shylock-Hg]

There will be consistency issue, if rmw operation isn't atomic.