MySQL 5.7

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select version();


version()


5.7.39


create table Cards_in_decks (id serial, player_id int);
insert into Cards_in_decks (player_id) values (1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1);
insert into Cards_in_decks (player_id) select 2 from Cards_in_decks;

Records: 40  Duplicates: 0  Warnings: 0



Records: 40  Duplicates: 0  Warnings: 0


select player_id,count(*) from Cards_in_decks group by 1;


player_id
count(*)


1
40

2
40


create table Cards_in_game_decks like Cards_in_decks
insert into Cards_in_game_decks select * from (
  (select * from Cards_in_decks where Cards_in_decks.player_id=1 order by rand() limit 10)
  union all
  (select * from Cards_in_decks where Cards_in_decks.player_id=2 order by rand() limit 10)
) random_cards

Records: 20  Duplicates: 0  Warnings: 0


select * from Cards_in_game_decks;


id
player_id


3
1

10
1

16
1

21
1

23
1

24
1

32
1

33
1

35
1

38
1

41
2

53
2

55
2

56
2

61
2

62
2

64
2

66
2

74
2

75
2