I see that isend on last message is "1", if this signifies the last message then you could use this to merge all objects and complete join when isend equals "1"
e.g.
[{"id":"2c3a94cf.7d5cec","type":"inject","z":"bf9e1e33.030598","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[{\"id\":\"3701211100108\",\"ua\":231.59,\"ub\":0,\"uc\":0,\"ia\":0,\"ib\":0,\"time\":\"20220322202100\",\"isend\":\"0\"},{\"id\":\"3701211100108\",\"ic\":0.04,\"uab\":232.028,\"ubc\":0,\"uca\":232.028,\"pa\":0,\"time\":\"20220322202100\",\"isend\":\"0\"},{\"id\":\"3701211100108\",\"pb\":0,\"pc\":0,\"zyggl\":0.0009,\"qa\":0.0073,\"qb\":0,\"time\":\"20220322202100\",\"isend\":\"0\"},{\"id\":\"3701211100108\",\"qc\":0,\"zwggl\":0.002,\"sa\":0.0035,\"sb\":0,\"sc\":0,\"time\":\"20220322202100\",\"isend\":\"0\"},{\"id\":\"3701211100108\",\"zszgl\":0.0049,\"pfa\":0.621,\"pfb\":0,\"pfc\":0,\"zglys\":0.621,\"f\":49.93,\"time\":\"20220322202100\",\"isend\":\"0\"},{\"id\":\"3701211100108\",\"unb\":0.999,\"inb\":0,\"pdm\":-0.0004,\"qdm\":0.0003,\"sdm\":0.0015,\"ig\":2039.708,\"time\":\"20220322202100\",\"isend\":\"1\"}]","payloadType":"json","x":130,"y":60,"wires":[["db4a76e4.7f2978"]]},{"id":"db4a76e4.7f2978","type":"split","z":"bf9e1e33.030598","name":"simulate incoming messages","splt":"\\n","spltType":"str","arraySplt":1,"arraySpltType":"len","stream":false,"addname":"","x":370,"y":60,"wires":[["38184dee.eb7bb2"]]},{"id":"38184dee.eb7bb2","type":"switch","z":"bf9e1e33.030598","name":"","property":"payload.isend","propertyType":"msg","rules":[{"t":"eq","v":"1","vt":"str"},{"t":"else"}],"checkall":"true","repair":false,"outputs":2,"x":190,"y":100,"wires":[["64329005.9ef378"],["cda82e63.9c89a"]]},{"id":"64329005.9ef378","type":"change","z":"bf9e1e33.030598","name":"","rules":[{"t":"set","p":"complete","pt":"msg","to":"true","tot":"bool"}],"action":"","property":"","from":"","to":"","reg":false,"x":410,"y":100,"wires":[["cda82e63.9c89a"]]},{"id":"cda82e63.9c89a","type":"join","z":"bf9e1e33.030598","name":"","mode":"custom","build":"merged","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","accumulate":false,"timeout":"","count":"39","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":170,"y":160,"wires":[["cb5dd3ae.255a3"]]},{"id":"cb5dd3ae.255a3","type":"debug","z":"bf9e1e33.030598","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":580,"y":160,"wires":[]}]
You then have all the data in one message and can construct your sql query.
p.s.
The MQTT node can output a parse json object if you select it in the config, no need for json node.